Frequency

As noted above, ultrasound transducers are designed to operate at a certain frequency or over a range of frequencies. Lower-frequency sound waves penetrate tissues better but cannot achieve the same resolution as higher-frequency probes. However, if too high a frequency for a certain patient is used, the lack of penetration severely degrades the images. Thus the highest frequency probe that allows adequate penetration should be used. This usually depends on the thickness of the patient’s abdominal wall. For obese patients, a lower-frequency probe must be chosen. The frequency used for most general purpose transabdominal obstetric probes is about 3 to 5 MHz. A lower-frequency transducer (2 to 2.25) may be needed to provide adequate resolution in obese patients. When available, a higher-frequency probe that operates at 5 MHz or above is useful in selected patients. Higher-end probes now operate at a range of frequencies, thus one transducer may operate at 2 to 5 MHz and another at 5 to 8 MHz. Because penetration of the maternal abdominal wall is not an issue, transvaginal probes usually operate at a frequency of 5 to 10 MHz. Most quality ultrasound machines now have a feature called tissue harmonic imaging . With this modality, a standard frequency (e.g., 3 MHz) is transmitted and propagated in the usual manner. Because a relatively low-frequency wave is emitted, good penetration is preserved. However, in the receive part of the cycle, the ultrasound machine listens for the reflection of the first harmonic wave (e.g., 6 MHz). This higher-frequency wave only has to travel one direction, thus some of the resolution benefit of high-frequency scanning is retained. This process also reduces noise in the image by removing various forms of artifact, making cystic structures appear free of echoes. Harmonic imaging often significantly improves image quality and should be used liberally when available.

Power

Ultrasound machines have the capability of delivering varying amounts of voltage to the transducer elements. Increasing the power output increases the amplitude of energy waves in the ultrasound beam and results in stronger returning echoes. This can improve the signal-to-noise ratio and can improve imaging capabilities. However, energy is required to create oscillations in the molecules of insonated tissues. Because this energy is absorbed into the tissues, delivering an unnecessarily high energy dose raises safety concerns. The safety of diagnostic ultrasound will be discussed later in this chapter.

Gain

Signals from the weak echoes returning to the transducer elements must be amplified before being used for display . This amplification process is referred to as gain . Because of attenuation, echoes returning from deeper within the body have a lower intensity, and the machine must boost the amplification from these echoes. This built-in processing feature is known as time-gain compensation (TGC), meaning that echoes with a greater time delay automatically have greater amplification. The amount of amplification, or gain, can be controlled by the sonographer in two ways, with the gain control knob or the TGC controls. The gain control knob adjusts the overall gain up or down, so the brightness of the image can be changed to optimize visualization of anatomic details. When an image is too bright or too dark, much diagnostic information is lost ( Fig. 9-1 ). Because tissue characteristics vary from patient to patient, penetration of sound waves at different depths may vary, so the gain at different depths may be adjusted. This is done using the TGC controls, a set of sliders on the instrument control panel ( Figs. 9-2 and 9-3 ). A uniform scale across the brightness values should be sought.

A, In this image the overall gain is too low, yielding a dark image. B, This image has too much gain. In both cases, diagnostic detail is lost.

A shows the slide bars that comprise the time-gain compensation controls. In this case, the fact they are lined up in the center indicates that no adjustment for different depths was needed (B) .

Incorrect time-gain compensation settings (A). The slide bars are inappropriately shifted to the left in the near field, making the corresponding area of the image too dark (B).

Attenuation

Attenuation of ultrasound waves is affected by the medium through which the sound waves pass. Virtually no pulses pass through gas. This is why there must be a coupling agent (e.g., ultrasound gel) applied between the transducer face and the patient’s skin. For several reasons, ultrasound waves lose intensity as they pass through tissues. The pressure waves gradually diverge from the central beam and are scattered by reflection from small structures within the tissue, and part of the sound energy is absorbed within tissues. Some tissues, such as bone, strongly attenuate sound waves. The thicker the tissues through which sound waves must pass before arriving at the target, the more the attenuation and the greater the difficulty in retrieving good information from the echoes. Because of attenuation, obstetric ultrasound imaging is greatly affected if the patient is obese. In patients with a thick, dense abdominal wall, image quality is greatly reduced. In such patients, attention to equipment controls and scanning technique is essential.

Focus

A linear array transducer makes an ultrasound beam by firing a row of crystals placed along the surface of the probe. When adjacent crystals fire, the pressure waves reinforce one another by a process called constructive interference . This phenomenon creates the central ultrasound beam that extends out from the probe. Electronic control of the timing and order that crystals are activated can work to focus this beam at the region of interest within the tissues. Image resolution is optimal when the structure of interest lies within this zone of optimal focus, which can be adjusted by the sonographer.

Depth and Zoom

When scanning, the sonographer should strive to cone down the view to best demonstrate important structures without filling the screen with irrelevant material. The depth control can be used to simply crop extraneous structures at the bottom of the image. The zoom control is a little more sophisticated; it magnifies a box within the image rather than just removing information from the bottom of the image ( Figs. 9-4 and 9-5 ).

Ultrasound image before zoom is applied. The area within the dotted lines should be expanded to fill the entire screen.

Appearance after zoom is applied. Compared with Figure 9-4 , the cardiac structures are more easily seen. Additionally, because a smaller area was being scanned, the frame rate increased from 25 to 58 frames per second.

Proper depth and zoom are important for several reasons. Most importantly, limiting the size of the scanned area allows a higher frame rate and resolution. Additionally, homing-in on the area of interest draws attention to important detail within that scanned area.

M-Mode

For most obstetric applications, the familiar 2-D gray-scale real-time ultrasound is used. This is formally known as B-mode ultrasound. Another ultrasound modality that is available on most machines is referred to as M-mode ultrasound (“motion mode”). M-mode ultrasound shows changes along a single ultrasound beam over time. The depth of the echo-producing structures is shown on the y-axis, and time is shown on the x-axis. M-mode is useful for documenting the presence of fetal cardiac activity ( Figs. 9-6 and 9-7 ), and M-mode is also sometimes used for specialized echocardiography applications.

M-mode application in an 8-week fetus. The row of dots in the left panel indicates the line of information, in this case cardiac pulsations, being displayed over time in the right panel. M-mode is preferred to Doppler for documenting fetal viability before 10 weeks. Note the prominent brain vesicle in the fetal head, a normal finding.

M-mode ultrasound with the cursor through the heart valves. In any ultrasound exam, it is important to have a permanent record showing fetal viability. M-mode is convenient for this purpose and can be used to determine the fetal heart rate.

Color and Pulse-Wave Doppler

Over the past 25 years, Doppler ultrasound imaging has assumed a key role in obstetrics. With this modality, the ultrasound machine detects shifts in the frequency of echoes returning from a specific location in the image. This frequency shift, the Doppler shift, is caused by motion of the insonated material toward or away from the transducer. Doppler ultrasound is primarily used to demonstrate the presence, direction, and velocity of blood flow. The machine displays moving blood as color superimposed on the 2-D gray-scale image. By convention, flow toward the ultrasound transducer is displayed in red and flow away is displayed in blue. Pulse-wave Doppler continuously measures the relative velocity of flow within a designated gate inside a vessel. Flow velocity waveforms are used to calculate the systolic/diastolic (S/D) ratio, the pulsatility index, and the resistance index.

These indexes are primarily used to assess downstream resistance in the vessel being interrogated . In pregnancies with fetal growth restriction, the flow within the umbilical artery is used to assess placental function ( Fig. 9-8 ). For some applications, the absolute flow velocity is needed. For example, when screening for fetal anemia, the peak flow velocity in the fetal middle cerebral artery is measured, as this correlates with the degree of fetal anemia . To give meaningful results, it is absolutely essential that the angle of insonation (θ) must be in line with the direction of blood flow ( Fig. 9-9 ). Most ultrasound machines are equipped with the technology to allow the sonologist to autocorrect the angle of insonation when the optimal angle cannot be obtained.

Color and spectral Doppler evaluation of the umbilical artery. In the left panel, the coiling arteries and vein are shown. Red indicates flow toward the transducer and blue is flow away. The sample gate for the pulse Doppler is superimposed. On the right is the result of the pulse Doppler, depicting a normal flow velocity waveform.

Color and spectral Doppler interrogation of the middle cerebral artery. Abnormally high peak velocity is an indicator of moderate to severe fetal anemia. Because the actual velocity is being measured, the angle of insonation should line up with the vessel.

Three-Dimensional Ultrasound

High-performance computers have allowed the development of ultrasound machines and probes that can acquire, process, and display a three-dimensional (3-D) volume, as opposed to the single plane displayed with 2-D ultrasound. To obtain this volume, the transducer uses an internal mechanical sweep mechanism that summates contiguous 2-D planes. This volume data can either be stored for analysis or updated and displayed on a continuous basis. Adding a real-time updating of a rendered image is commonly referred to as four-dimensional ultrasound.

For diagnosing certain birth defects, 3-D ultrasound may be useful. Information from an acquired volume may be processed in such a way that the fetal surface is displayed in a lifelike manner. Surface abnormalities, such as facial clefts, can be well demonstrated with this approach ( Fig. 9-10 ). In addition, 3-D images can be more readily understood by patients and other professionals who will participate in care of the baby.

Three-dimensional image shows bilateral cleft lip. Images such as this can be helpful for counseling patients.

Software available for use with 3-D ultrasound machines can manipulate stored volume data off-line to show any desired plane through the scanned area. Some of these planes may be difficult to obtain with standard 2-D imaging. Storage of a volume of data also allows retrospective generation of 2-D images from different planes than were originally recorded. This could be a powerful tool for review of ultrasound exams. Another feature of 3-D ultrasound is its ability to calculate tissue and fluid volumes. For example, lung volume measurements have been used to predict pulmonary hypoplasia.

Despite the demonstrated capabilities of 3-D ultrasound, no proof exists of an advantage of this technology over standard 2-D imaging for prenatal diagnosis. A 2009 American College of Obstetricians and Gynecologists (ACOG) practice bulletin states that “three-dimensional ultrasonography may be helpful in diagnosis as an adjunct to, but not a replacement for, two dimensional ultrasonography.”

Orientation

The sonographer starts the exam by exposing the patient’s skin over the entire uterus and liberally applying warmed coupling gel. An appropriate probe is selected, and a preliminary survey of the entire uterine contents and adnexa is performed. Before going any further, it is a good idea to document fetal cardiac activity . Images are frozen and recorded liberally. Almost all machines have a cine loop feature in which a few seconds of sequential images are saved so that the sonographer can scroll backward if the desired image was missed.

Every effort should be made to perform the scan with the sonographer and the patient in standard positions. In most settings, the ultrasound machine and the sonographer are on the patient’s right, with the sonographer comfortably seated facing the head of the patient. The probe is held in such a way that the image on the screen is properly oriented. By convention, the probe surface is shown at the top of the screen. For sagittal views, the right of the screen corresponds to the inferior aspect of the patient ( Fig. 9-11 ). For transverse views, the patient’s right is shown on the left of the screen. With transvaginal ultrasound, transverse views also show the patient’s right side to the left of the screen. In transvaginal sagittal views, “up” is to the left of the screen (i.e., toward the bladder), and “down” (toward the sacrum) is to the right of the screen ( Fig. 9-12 ). Ultrasound transducers have a notch or ridge that demarcates the side of the probe that will correspond to the left side of the monitor. Thus with transabdominal work, this mark would be toward the patient’s head for sagittal views and toward the patient’s right for transverse views. With transvaginal scanning, the mark is up for sagittal views and to the patient’s right for transverse views. Thus in going from sagittal to transverse, the probe is always rotated counterclockwise, and the clinician goes clockwise to move from transverse to sagittal.

Transabdominal sagittal view of the uterus. The uterine fundus and cervix are labeled. By convention, the right side of the ultrasound screen corresponds to the inferior aspect of the patient.

Transvaginal sagittal view showing proper orientation. The left of the screen is “up” on the patient, that is, toward the bladder (Bl). The probe tip (Pr), fetal head (FH), cervix (Cx), placenta (Pl), and rectum (R) are labeled. Note how transvaginal ultrasound provides the ultimate “window,” showing very clear views of structures close to the vaginal apex. Also note the presence of vasa previa.

If the sonographer sits or stands on the wrong side of the patient or holds the probe backward, standard orientation of the images is difficult to maintain. A “backward” image is clearly unacceptable for diagnostics and for documentation. Also, a casual approach to probe orientation will prevent the sonographer from developing the hand-eye coordination needed to quickly and accurately steer the probe.

To establish the position of the fetus, the orientation of the probe must obviously be correct. This is important not only when deciding whether the fetus is cephalic or breech but also when determining the right and left side of the fetus. For example, when the image shows the fetal spine to the right side of the uterus, the fetal left will be up in a cephalic presentation and down with a breech. The sonographer should not depend on the side of the stomach or axis of the heart to define the position of the fetus because these structures are not always in a normal position.

Angle of Insonation

There is often a best angle from which to view aspects of the intrauterine contents and fetal structures. For example, transverse views of the kidneys are best obtained from a directly anterior or posterior direction. With a lateral view, one kidney is shadowed by the spine ( Fig. 9-13 ). Sliding the transducer across the mother’s abdomen to change the angle of insonation by 90 degrees corrects this problem. Sometimes, significant pressure with the transducer is needed to get the probe into position for optimal visualization. If the fetal position precludes clear visualization of a structure, a good strategy is to move forward with the examination and come back to the troublesome area. The fetus will often have moved in such a way that adequate views can be obtained.

A, Shadowing by the spine precludes visualization of the left kidney. B, By sliding the transducer to a different location on the maternal abdominal wall, a more favorable angle of insonation is possible. The spine (Sp), right kidney (RK), and left kidney (LK) are labeled.

Using Natural Windows

As previously noted, increasing the power of ultrasound impulses can overcome some of the effects of attenuation in maternal tissues. Although it is tempting to use higher-power settings for obese patients, other methods should be attempted first. As a starting point, the proper probe frequency and gain setting should be used. Even more effective, however, is to avoid attenuation altogether by scanning though one of the natural “windows” in the maternal abdominal wall. The abdominal wall thickness in obese women is substantially less near the umbilicus ( Fig. 9-14 ) and in the suprapubic area. To a lesser extent, thickness is also decreased lateral to the central pannus. Using these windows can often improve the quality of images dramatically. Of course, in early pregnancy or when structures of interest are low in the pelvis, the problem of attenuation is reduced considerably by the use of an endovaginal probe (see Fig. 9-12 ). The reduction of attenuation with transvaginal ultrasound allows the use of a higher-frequency probe, which usually results in excellent resolution. Another natural window is amniotic fluid, and scanning through amniotic fluid improves the image quality below or deep to the amniotic fluid. This is especially true for imaging the surface of the fetus. A full bladder provides a window to structures low in the maternal pelvis. However, scanning with a full maternal bladder has significant disadvantages; not only is it uncomfortable for the patient, it also artificially elongates the cervix and may distort the apparent relationship of the cervix and placenta. Fortunately, this is no longer necessary because transvaginal ultrasound can provide superior views to structures in the maternal pelvis.

A, Scan through the pannus of the abdominal wall. B, By moving the probe into the relatively thinner area near the maternal umbilicus, the resolution improved dramatically. A similar result can be obtained when scanning below the pannus in the suprapubic area. Of course, when an object of interest is in the maternal pelvis, transvaginal ultrasound provides an even better window (see Fig. 9-12 ). This fetus has truncus arteriosus (Tr).

First-Trimester Normal Findings

Both ACOG and the American Institute of Ultrasound Medicine (AIUM) have defined the essential components of a first-trimester scan. Knowledge of the time at which embryonic structures normally appear is important for identifying pathologic pregnancies. For the reasons noted earlier, it will be assumed that transvaginal ultrasound is being used for this discussion.

The gestational sac can usually be seen at 4 weeks, the yolk sac at 5 weeks ( Fig. 9-15 ), and the fetal pole with cardiac activity by 6 weeks. Cardiac activity can be seen simultaneously with the appearance of a fetal pole as a pulsation at the lateral aspect of the yolk sac. Because pulse or color Doppler carry higher energy, which theoretically could be harmful during embryogenesis (before 10 menstrual weeks), M-mode ultrasound is used to document cardiac activity during this time (see Fig. 9-6 ). Starting at 7 weeks, the embryo has grown to the point that recognizable features, such as a cephalic pole, can be seen. As shown in Figure 9-6 , a prominent midline brain vesicle can be seen at this time. The cerebral falx is visible at 9 weeks, and the appearance and disappearance of physiologic gut herniation are noted between 8 and 11 weeks ( Fig. 9-16 ). In the course of this physiologic process, the bowel is seen to lie within the umbilical cord and does not float freely. Obviously, the diagnosis of an abdominal wall defect should be made with caution at this age. The stomach can consistently be seen by 11 weeks. If conditions are favorable, it is often possible to also visualize the bladder and kidneys at 11 weeks. At about 12 weeks, using color Doppler, the two umbilical arteries can often be identified as they course around the bladder. The fetal heart rate is initially quite slow, averaging 110 beats/min at 6 weeks, then it increases steadily to a mean peak of 157 beats/min at 8 weeks. When the fetal position is favorable, transvaginal ultrasound has the potential for giving good views of the fetal cardiac anatomy in most patients at 13 weeks. Until 13 to 16 weeks’ gestation, the amnion has not fused to the chorion and is seen as a separate membrane (see Fig. 9-16 ). Until 12 weeks, the crown-rump length (CRL) should be measured for gestational age determination. Care should be taken to measure the full length of the fetus. The gestational age can be significantly underestimated if an oblique plane is used ( Fig. 9-17 ).

This image shows a normal 7-week gestational sac with the yolk sac (YS) and adjacent fetal pole (FP). Calipers show the measurement to establish the gestational age. CRL, crown-rump length.

Nine-week fetus showing the physiologic gut herniation ( curved arrow ) . Note also how the amnion ( straight arrow ) has not yet fused to the chorion at the uterine wall.

A, Image of the fetus is cut obliquely, and the crown-rump length is inappropriately short. It is measured correctly in B. The difference in the calculated gestational age from these two measurements was 5 days. The normal brain vesicle (asterisk) is noted.

First-Trimester Abnormal Findings

Spontaneous abortion occurs in 15% of clinically established pregnancies. When cardiac activity has been demonstrated, the miscarriage rate is reduced to 2% to 3% in asymptomatic low-risk women. It is important to note, however, that in some groups at high risk for miscarriage—such as women over the age of 35 years who are undergoing infertility treatments—early visualization of cardiac activity does not provide quite as much reassurance. In one study that involved such women, the miscarriage rate in asymptomatic women was still 16% after a heartbeat was documented. In younger women who present with bleeding, only 5% miscarry if the ultrasound is normal and shows a live embryo. If an intrauterine clot is present ( Fig. 9-18 ), coexistent with an otherwise normal-appearing pregnancy, the miscarriage rate is 15%.

This patient presented with vaginal bleeding. A subchorionic clot (Cl) was present in the lower uterine segment. This patient carried the pregnancy successfully. The fetus (Fe) and placenta (Pl) are shown.

In the majority of pregnancies destined to abort, the embryo does not develop, and ultrasound shows an empty gestational sac ( Fig. 9-19 ). Such a pregnancy is termed an anembryonic gestation . When a failed pregnancy is suspected based on clinical or sonographic grounds, patients and clinicians alike are anxious to determine viability as soon as possible. However, the potential for great harm is obvious if a pregnancy is incorrectly deemed to be a failed pregnancy, and a desired pregnancy is interrupted. There have been cases in which a premature diagnosis of a failed pregnancy was made, medical evacuation of the uterus was unsuccessfully attempted, and the fetus was subsequently found to be viable. Because no significant medical risk attends waiting for certainty when a failed pregnancy is suspected, a cautious approach is always advisable. Criteria for deciding that a pregnancy of uncertain viability is in fact a failed pregnancy should be set to virtually eliminate the possibility of a false-positive diagnosis. A multispecialty panel from the Society of Radiologists in Ultrasound panel recommended criteria to achieve this end. These include (1) the presence of a fetus with a CRL of more than 7 mm and no heartbeat, (2) the absence of an embryo when the mean sac diameter is greater than 25 mm, (3) the absence of an embryo with a heartbeat more than 2 weeks after a scan showed a gestational sac without a yolk sac, and (4) the absence of an embryo with a heartbeat more than 11 days after a gestational sac with a yolk sac was seen. Similar but less stringent criteria that are suspicious for a pregnancy failure are described in the same report. Other signs that indicate a possible failed pregnancy include an enlarged yolk sac (>7 mm; Fig. 9-20 ) or less than 5 mm difference between gestational sac diameter and the CRL of the embryo. If there are borderline findings and uterine evacuation is being considered, it is prudent to repeat the ultrasound in 7 to 10 days to be absolutely sure that a viable pregnancy is not interrupted. Although a quantitative human chorionic gonadotropin (hCG) value that does not show an appropriate rise may indicate an abnormal pregnancy, a decision to medically or surgically evacuate an intrauterine failed pregnancy should be based on abnormal ultrasound findings.

Irregular gestational sac from an anembryonic gestation.

Compared with Figure 9-15 , the yolk sac in this image is relatively large for the gestational age. An embryo was never seen with this pregnancy.

First-trimester ultrasound findings predictive of a chromosome abnormality include a thick nuchal translucency, absent nasal bone, abnormally fast or slow fetal heart rate, and some structural malformations. The first-trimester aneuploidy screen will be discussed in detail in Chapter 10 .

Types of Examinations

The AIUM, in conjunction with ACOG and the American College of Radiology (ACR), have defined a set of criteria for standard obstetric ultrasound examinations performed in the second and third trimesters. Components of a standard obstetric examination are shown in Box 9-1 . A complete description of the AIUM and ACOG guidelines can be found in the listed references.

These guidelines recognize that not all ultrasound examinations have the same purpose. For this reason, types of fetal sonographic evaluations have been defined. Components of the first-trimester ultrasound examination were described previously. A standard second- or third-trimester examination (current procedural terminology [CPT] code 76805), as defined in Box 9-1 , can be performed by any appropriately qualified sonographer. It is recognized that certain scans, termed specialized examinations (CPT code 76811), are more complex than the complete standard examinations performed in the course of routine pregnancy care. This designation and billing code are intended to be used for referral practices with special expertise in the identification of and counseling on fetal anomalies. Other specialized examinations may include fetal Doppler ultrasonography and fetal echocardiography. Follow-up examinations are needed for many obstetric conditions; these are termed repeat examinations (CPT 76816).

Another examination category is the limited examination (CPT 76815). Limited ultrasound examinations, also performed by a trained sonographer, are used to obtain a specific piece of information about the pregnancy. Examples of this type of examination include the determination of fetal lie or assessment of amniotic fluid volume. The importance of restricting limited examinations to those cases in which a complete examination has previously been performed should be self-evident. Consider the consequence if a brief ultrasound is done and critically important information is missed, such as a serious malformation in the fetus. Unfortunately, it is all too common for practitioners to perform limited examinations in a manner inconsistent with good medical practice. For example, in some clinics, practitioners perform an ultrasound at the first prenatal visit to document viability but do not measure the fetus or record the results of the examination. Such a practice can create problems later in pregnancy when the gestational age is in doubt.

All the aspects of the standard obstetric examination listed in Box 9-1 are important for clinical management and should not be neglected. It is clearly unacceptable for an ultrasound exam to miss such conditions as placenta previa, multiple gestation, or an ovarian tumor. The diagnosis and management of these and other conditions are discussed in detail elsewhere in this book, but a brief description of the importance of the components of the standard ultrasound examination will be given here.

Qualifications for Performing and Interpreting Diagnostic Ultrasound Examinations

Most ultrasound exams in the United States are performed by professionals who are credentialed by the American Registry of Diagnostic Medical Sonography (ARDMS). Individuals with these credentials have had extensive education and testing to ensure their competency. A physician then generates a report based on the images and information obtained by the sonographer. When appropriate, the physician may personally perform or repeat parts of the exam. The AIUM has published guidelines on the training and experience needed for physicians to perform or interpret ultrasound examinations. In brief, these guidelines recommend that licensed physicians have completed an equivalent of 3 months training dedicated to ultrasound in the context of an approved residency, fellowship, or other postgraduate training. In the absence of a formal training program, physicians can qualify through having 100 American Medical Association (AMA) category I credits dedicated to diagnostic ultrasound. In addition to participation in either a formal training program or by taking postgraduate courses, physicians should have been involved with the performance, evaluation, and interpretation of at least 300 appropriately supervised sonograms. The full text of the guidelines is in the referenced document.

Cardiac Activity

Obviously, the presence or absence of cardiac activity should be documented. As noted previously, after about 6 weeks’ gestation, the diagnosis of fetal life is rarely difficult. Even though fetal death may be obvious with B-mode imaging, confirming the absence of a heartbeat with color or pulse-wave Doppler is recommended. Absence of color signal in the fetal chest, contrasted with the demonstration of flow in the surrounding uterine tissues, can increase the confidence that fetal death has indeed occurred. Throughout pregnancy, an abnormally fast, slow, or irregular heartbeat can be detected by visual inspection with gray-scale ultrasonography. The abnormal rate can be quantified and documented with M-mode or pulse-wave Doppler ultrasound.

Number of Fetuses

When a multiple pregnancy is diagnosed, the number of amnions and chorions should always be determined (see Chapter 32 ). Determination of chorionicity is most easily accomplished in early pregnancy. The presence of unlike sex twins, separate placentae, or a thick membrane dividing the sacs with a twin peak or “lambda sign” all indicate the presence of two chorions (see Fig. 9-3 ). It is well-recognized that the level of fetal risk is much higher when fetuses share chorions, and the risk is extremely high if there is a single amnion. Monochorionic pregnancies require early referral for a specialized ultrasound. In all twin pregnancies, periodic ultrasound examinations should be performed to assess fetal growth. Twins are at significantly increased risk for growth abnormalities, and it is not possible to assess the growth of the twins individually by abdominal palpation.

Presentation

The assessment of presentation is not merely a matter of determining whether the fetus is head down or breech. A more precise ultrasound analysis of presentation is important in certain circumstances. If a transverse lie is diagnosed, it is important to diagnose whether the fetus is back down (i.e., back toward the cervix) because this may require a vertical incision for cesarean delivery. If the patient has preterm labor or ruptured membranes, a back-up transverse lie indicates a high risk for cord prolapse. The attitude of the fetal head, especially a face presentation, can be important in assessing progress in labor. In cases of marked caput or molding in late labor, it is often difficult to determine the position of the fetal head by palpation of the cranial sutures. Under these circumstances, ultrasound can be used to readily identify fetal cranial landmarks to clarify the position of the fetal head.

Amniotic Fluid Volume

Every ultrasound examination should include an assessment of the amniotic fluid volume (see Chapter 35 ). It is acceptable for an experienced examiner to make this determination subjectively. However, to aid in communication and to provide criteria for management protocols, semiquantitative methods have been devised. A popular method is the amniotic fluid index (AFI), the sum of the measurements of the deepest vertical pocket of fluid in each of the uterine quadrants ( Fig. 9-21 ). The limits of the quadrants are the maternal midline and a horizontal line through the maternal umbilicus. Each pocket should measure at least 1 cm in width. The line between the calipers should not cross through loops of cord or fetal parts. Polyhydramnios and oligohydramnios can be defined either by an AFI outside of a fixed range, usually defined as greater than 24 cm or less than 5 cm, respectively. A simpler semiquantitative method is to diagnose polyhydramnios when the single deepest pool measures greater than 8 cm ( Fig. 9-22 ) and oligohydramnios when the shallowest pool measures less than 2 cm in two dimensions .

These sonographic images show the deepest vertical pocket in each of the four quadrants of the uterus. The sum of the measurements of these pockets is the amniotic fluid index.

An ultrasound image showing polyhydramnios. The distance between the anterior and posterior uterine walls was 9 cm.

The actual volume of amniotic fluid can be determined by dye-dilution techniques performed at the time of amniocentesis. The semiquantitative ultrasound methods described previously correlate somewhat with actual fluid volume, but their accuracy for predicting abnormal fluid volume is limited.

Placenta and Umbilical Cord

One of the principal advantages of routine ultrasound is that serious problems of placentation—such as placenta previa, placenta accreta, and vasa previa—can be diagnosed in a timely manner (see Chapter 18 ). At the time of the routine screening ultrasound (beyond 18 weeks), it should be determined whether the placenta covers the internal cervical os. If the placenta and the cervix are not seen clearly, or if it appears that the edge of the placenta is close to the cervix, vaginal ultrasound should be used liberally to clarify this relationship ( Figs. 9-23 and 9-24 ).

Sagittal transabdominal view of the lower uterus and cervix. The relationship between the edge of the placenta and cervix is unclear, and placenta previa cannot be accurately diagnosed.

Sagittal transvaginal view of the same patient. Marginal placenta is clearly visible, and the edge of the placenta (Plac edge) extends 1.5 cm over the internal os (Int cx os). Calipers are not shown.

The terms complete and partial placenta previa originated from a time when the relationship of the placenta to the partially dilated cervix was determined by digital examination. Because the internal cervical os is not typically dilated at the time of ultrasound, these definitions are confusing and often uninformative. For this reason, a joint statement by the major ultrasound and obstetric societies in the United States and a policy statement by the Society of Obstetricians and Gynaecologists of Canada recommended a classification that retains only the terms placenta previa and low-lying placenta . The distance that the edge of the placenta covers or ends short of the internal cervical os should be measured and reported. This quantitative description is much more helpful than a report of “complete” or “partial” previa for predicting the future placental position and in planning management. For pregnancies greater than 16 weeks, if the placental edge ends 2 cm or more from the cervix, the placental location should be reported as normal. If the placental edge is less than 2 cm from the internal os but not covering the internal os, the placenta should be labeled as low lying, and follow-up ultrasonography is recommended at 32 weeks’ gestation .

Between 18 and 23 weeks’ gestation, the edge of the placenta extends to or covers the internal os of the cervix in about 2% of patients. However, most cases of placenta previa diagnosed early in pregnancy resolve as pregnancy progresses. The rate of persistence of placenta previa depends on the degree of overlap. When the degree of overlap is 15 mm or greater, 19% persist as placenta previa, whereas if the overlap is 25 mm or greater, 40% remain. Another study found that when placenta previa is present at 15 to 19 weeks, only 12% persist. The rate of persistence gradually increases as the gestational age advances, up to 73% if placenta previa is present at 32 to 35 weeks. This study also showed that the degree of overlap was helpful in predicting persistence. These results suggest that repeated ultrasound examinations should be performed until the placenta moves well away from the cervix or until it becomes clear that the previa will persist. If the placenta previa persists, ultrasound can be very valuable in planning delivery.

The diagnosis of vasa previa is critical because the recognition of this finding at the time of a screening ultrasound greatly affects the chance of fetal survival. The fetal mortality rate is high when vasa previa is not diagnosed before labor. Conversely, early diagnosis and aggressive obstetric management of patients with vasa previa almost always results in a live baby born in good condition. The fetal vessels that cover the cervix may not be readily apparent with a routine transabdominal screening examination; therefore a high index of suspicion should be maintained. Transvaginal color Doppler ultrasound should be strongly considered in any case of velamentous cord insertion, a succenturiate lobe, or when portions of umbilical cord are noted to be low in the uterus ( Fig. 9-25 ). The sonographer should also be aware that when placenta previa “resolves,” branches of the umbilical vessels on the chorionic plate may still course over the cervix as placental villi degenerate beneath them, resulting in vasa previa. Identifying the cord insertion onto the placenta eliminates the possibility of velamentous cord insertion, but it does not exclude vasa previa from the other placentation abnormalities. Documentation of the cord insertion onto the placenta is good practice, and it is recommended when technically possible.

Sagittal transvaginal image showing vasa previa. An umbilical vessel (UV) passes over the internal os of the cervix (Cx). In this case, a velamentous cord insertion was evident, and this vessel passed over the cervix running from the cord insertion to the placenta.

In addition to determining the placenta’s location, its appearance should be assessed. Many changes observed in the placenta are related to calcification, fibrosis, and infarction. The general trend is for these changes to become more apparent as pregnancy progresses, but their clinical significance is unclear. It has recently been recognized that a “globular” placenta, with a narrow base compared with height, is associated with an increased rate of IUGR, fetal death, and other complications.

The sonographer should confirm that there are two arteries and a vein in the umbilical cord. In late pregnancy, this can be ascertained by looking at a transverse cut of the cord in a free loop. In the second trimester, two umbilical arteries are most easily confirmed by identifying the vessels with color Doppler as they course around the fetal bladder ( Fig. 9-26 ). A single umbilical artery is present in about 0.5% of all newborns. Because of the increased incidence of associated malformations, especially those that involve the kidneys and heart, this finding should prompt a detailed fetal survey. Fetuses with a single umbilical artery have a 20% chance of growth restriction. Additionally, the rate of polyhydramnios, abruption, placenta previa, structural placental abnormalities, cesarean delivery, low Apgar scores, and fetal death are all increased.

A, Umbilical vein (UV) and a single umbilical artery (UA) are shown in a transverse section of a free loop of cord. B, A transverse view of the fetal pelvis, using color Doppler, clearly shows the normal paired umbilical arteries coursing around the bladder (Bl). It is often easier to document the umbilical arteries in early pregnancy with this color Doppler method.

Uterus and Adnexa

With any obstetric ultrasound, including those performed in the first trimester, the adnexal and uterine morphology should be evaluated. Many women enter pregnancy without being aware that they have fibroids or a müllerian malformation. Fibroids are usually readily apparent with transvaginal or transabdominal ultrasound. A common pitfall is to confuse uterine contractions, which are commonly present in the second trimester, with fibroids ( Fig. 9-27 ). Contractions have a more lenticular shape and blend with the surrounding myometrium, whereas fibroids usually are spherical with distinct borders and a whorled internal echo texture. Pedunculated fibroids, even large ones, can be missed if the sonographer does not examine areas around the periphery of the uterus. Most studies have shown a higher rate of pregnancy complications when fibroids are present. However, the odds ratios are typically modest and of limited clinical importance. It is difficult to predict how fibroids will affect an individual patient’s pregnancy because the number, size, and location can vary markedly. Ultrasound mapping of fibroids can help in making a delivery plan. Although large fibroids that fill the pelvis may preclude vaginal birth, it is usually prudent to not predict early in the pregnancy the need for cesarean delivery. Fibroids can rise out of the pelvis, leaving a relatively clear lower uterine segment. Because multiple fibroids in the lower uterus can greatly complicate the performance of a low transverse cesarean delivery, ultrasound may help predict the need for a classical incision. Studies with serial ultrasound exams have had conflicting results in regard to the growth of fibroids during pregnancy.

A, Two fibroids are shown that are round and well circumscribed. B, Uterine contraction is lenticular in shape and does not have a clear border. Contractions such as this are very common in the second trimester.

Adnexa

It is important for a complete obstetric ultrasound to include an assessment of the adnexa. Normal-sized ovaries may be difficult or impossible to see in the second or third trimester because of shielding by bowel. In a study of sonographic visualization of normal ovaries, Shalev and coworkers found that although both ovaries were visible in almost all first-trimester scans, both ovaries were visible in only 16% of second- and third-trimester scans, and in 60% of these, neither ovary was seen. However, in most cases when a significant tumor is present, it will be visible. The sonographic appearance of an adnexal mass discovered during pregnancy guides decision making regarding the need for surgical removal. Masses that consist of simple cysts usually represent benign processes and do not require removal during pregnancy. Masses with features of malignancy—such as large size, multiple cystic cavities, thick septa, internal papillae, or solid areas—require careful evaluation and may require operative removal. The most common neoplasms in pregnancy are benign cystic teratomas ( Fig. 9-28 ). These can usually be identified by their sonographic characteristics.

Transabdominal and transvaginal images of an ovarian dermoid. A, An 8-week pregnancy with the uterus (Ut) demarcated in the transabdominal image. B, Using transvaginal ultrasound, the mass fills the entire field of view. Characteristic irregular mixed solid and cystic areas with highly echogenic nodules ( arrows ) are seen within the dermoid. To see the pelvic structures clearly with transabdominal ultrasound, the maternal bladder (Bl) must be very full. Although this provides a good window, the structures of interest are pushed away from the probe, which limits resolution. In contrast, the probe is less than 1 cm from the mass when vaginal ultrasound is used.

Anatomic Survey

Systematic evaluation of fetal anatomy is critical. It is a good idea to proceed with an examination in a consistent order so that important parts are not forgotten. Although many well-recognized maternal risk factors exist for congenital anomalies, 90% of birth defects occur in fetuses of low-risk women. For this reason, it is important that anyone who performs obstetric ultrasound have familiarity with the appearance of normal fetal anatomy in order to recognize deviations from normal. As mentioned previously, although more advanced detailed sonograms (specialized examinations) are appropriate for patients with identified risk factors, all standard examinations should include a full anatomic survey. An overview of some of the more common fetal malformations that can be recognized at the time of a standard ultrasound examination is presented later in this chapter.

When components of the standard second- or third-trimester ultrasound examination cannot be obtained because of maternal obesity, unfavorable fetal position, or other technical factors, this should be documented in the report. In such patients, it is reasonable to repeat the ultrasound in 2 to 4 weeks. If a second attempt is unsuccessful, no further exams to attempt better visualization are necessary.

Documentation

A report for any ultrasound exam should list the indication. Documentation of ultrasound findings is important not only for good patient care but also for quality review and legal defense. AIUM guidelines in regard to record keeping state, “Adequate documentation of the study is essential for high-quality patient care. This should include a permanent record of the sonographic images, incorporating whenever possible … measurement parameters and anatomic findings. ”

Cleaning and Disinfection of Probes

To avoid transmitting disease from one patient to another, proper cleansing and disinfecting of probes is important. With a transabdominal probe, it is sufficient to simply wipe the probe clean with a disposable antiseptic paper towelette. Transvaginal probes should be covered during use with a disposable latex or nonlatex cover. Following the exam, the probe should be cleaned with running water or a damp cloth. It is then important that chemical high-level disinfection be carried out as per the probe manufacturer’s recommendations.