The use of ultrasound in obstetrics has become a vital component in assessing the well-being of the fetus and fetal growth as well as in quickly evaluating common obstetric complications. Familiarity with the basic principles and several high-yield applications of obstetric ultrasound is an important component of the well-prepared obstetric and gynecologic (OB/GYN) hospitalist’s skill set, allowing a more complete, rapid assessment of patients in acute situations.

ULTRASOUND SAFETY

As with all modalities of imaging in the pregnant patient, the use of ultrasound abides by the ALARA principle (which stands for “as low as reasonably achievable”).¹ The American College of Obstetrics and Gynecology (ACOG) and the American Institute of Ultrasound in Medicine (AIUM) agree that the use of ultrasound during pregnancy is not associated with risk to either the fetus or the pregnant patient; however, ultrasound should be performed only by those trained to recognize medically important conditions and artifacts that may mimic pathology, and who also know techniques to avoid using ultrasound except when it is considered safe and necessary.

BASIC PRINCIPLES OF ULTRASOUND

Ultrasound is produced by sound waves that are emitted and then reflected back at varying densities among the uterus, placenta, amniotic fluid, and fetus in order to create images of them. Higher-density structures, such as bone, reflect sound waves at a greater velocity, causing them to appear hyperechoic (bright). Lower-density structures, such as amniotic fluid, reflect back fewer sound waves and at a lower velocity, causing them to appear anechoic or echolucent (dark).

Choosing an ultrasound transducer with a higher frequency (such as 4–7 mHz) results in better resolution; however, this comes at a cost to depth of penetration. One example of when a higher-frequency transducer may be beneficial would be to use a transvaginal ultrasound probe (typically 5–9 mHz) to assess cervical length because of the proximity of the specific structure being evaluated. In contrast, an ultrasound transducer with a lower frequency (such as 2–5 MHz) is beneficial in cases where increased depth of penetration may be needed (as in the evaluation of a morbidly obese pregnant woman). However, the resulting image will have less resolution than a higher-frequency transducer.

After selecting the most appropriate probe for a sonographic evaluation, the operator must be sure of the relationship between the right and left of the image on the screen and what is being evaluated. On all ultrasound transducers, there is a small notch or line on one side. By convention, this demarcation always should be on the patient’s right side or toward the patient’s head. When the transducer is held in a longitudinal axis (with the notch or line toward the patient’s head), it will produce a sagittal image, with the right side of the screen corresponding to that which is under the superior aspect of the transducer. When the transducer is held in a horizontal fashion (with the notch or line toward the patient’s right), it will produce a transverse image, with the right side of the screen corresponding to that which is underneath the right side of the transducer.

FETAL GROWTH AND WELL-BEING

The assessment of fetal growth should be approached in systematic manner (paying attention to the elements listed in Table 65-1). Each provider undoubtedly will develop a unique system, but it is the authors’ recommendation that each assessment perform the same steps each time.

Placental location should be described as anterior, posterior, or left or right lateral. It also may then be described as fundal, miduterine, low-lying, or previa. A placenta that appears low-lying (i.e. <2.0 cm from the internal os) on a transabdominal ultrasound may be differentiated from a placenta previa more easily by taking advantage of the higher frequency of a transvaginal probe.

Fetal presentation may be denoted as cephalic, breech, transverse, funic (umbilical cord presenting), or variable. Ultrasound may be used to further delineate whether a fetus in cephalic presentation is in an occiput anterior or occiput posterior position, which may be discerned by the ability to visualize fetal orbits in the case of the occiput posterior one. If funic presentation is suspected, the sonographer should take the time to differentiate between funic presentation and vasa previa, using transvaginal ultrasound if necessary (see the subsection “Vasa Previa,” later in this chapter).

Gestational Age

Gestational age and weight of a fetus can be determined by a number of various formulas or regression models. The most common of these is the model from Hadlock et al. (1985), in which estimated fetal weight and gestational age are determined by the fetal head circumference (HC), biparietal diameter (BPD), abdominal circumference (AC), and femoral diaphysis length (FL) in a regression model.² If any of these measurements is deemed suboptimal, a different formula or regression model should be selected so that only optimal measurements are included. For example, if the practitioner is unable to obtain a femur length, the Shepard et al formula (which utilizes AC and BPD) may be used²⁸. It is important to note that even with the most precise measurements, these models can have errors of about +/− 15% according to the AIUM.³

On occasion, the estimated gestational age by ultrasound may be discordant with the patient’s last menstrual period. ACOG provides an expert opinion⁴ for when it is recommended to redate a pregnancy in the event that a discrepancy is noted between the last menstrual period and the gestational age by ultrasound (Table 65-2).

The BPD is measured from a cross-sectional view at the level of the thalami and cavum septum pallucidi (Fig. 65-1A). Neither the orbits nor the cerebellum should be visualized in this transverse view. There should be a symmetric appearance of both hemispheres and a continuous midline echo created by the falx cerebri. The caliper should be placed on the outer edge of the proximal skull to the inner edge of the distal skull.

The occipital frontal diameter (OFD) is measured in the same cross-sectional view as the BPD. The calipers are placed on the outer edges of both the frontal and occipital bones. The OFD can be used in conjunction with the BPD to further describe the head shape by calculating the cephalic index (CI) with the formula BPD/OFD × 100. A normal CI is approximately 78% (50th percentile).

The term dolichocephaly describes an elongated head shape (CI < 70%, > 2 standard deviations). Brachycephaly describes a rounded head shape (CI > 86%, > 2 standard deviations). The presence of brachycephaly or dolichocephaly yields a less accurate BPD, and the HC should be used as a measurement of fetal growth in these cases.⁵ It is important to note that findings of dolichocephaly or brachycephaly may be normal variants, but they also can be secondary to positional changes or oligohydramnios, or they may be associated with abnormal findings (e.g. brachycephaly in Down syndrome).

The HC is measured at the same level as the BPD. This measurement depends upon the settings of each individual ultrasound machine, but it may be approximated from the BPD and OFD [HC = π(BPD + OFD)/2] or, more commonly, by using an ellipse function to outline the HC directly. If the ellipse function is used, it should measure the outer perimeter of the skull.

The cerebellum measurement is not taken into account when calculating estimated fetal weight using the Hadlock regression model; however, it can be useful for assessing gestational age, as cerebellar diameter is not affected by growth restriction.⁶ In cases where time is scarce and a determination of fetal age must be quickly established, there is some evidence to demonstrate that a BPD ≥ 9.3 cm with a Grade III placenta (i.e. significant calcifications and indentations of the chorionic plate) correlates with pulmonary maturity.⁷

The FL can be reliably used after 14 weeks gestation. The optimal view for measuring it is with the beam perpendicular to the shaft (Fig. 65-1B). The distal femoral epiphysis should not be included in this measurement.

The AC should be measured from the skin line in a transverse manner, with the umbilical vein, portal sinus, and fetal stomach in view (Fig. 65-1C). Again, an ellipse function may be utilized, or two sets of calipers 90 degrees apart using the equation AC = (distance 1 + distance 2) × 1.57 (i.e. π × the average of the two diameters). The kidneys and heart should not be seen at this level. The operator should try to achieve the most circular measurement possible. An AC that is small for gestational age may be the first finding of fetal growth restriction.

Fetal Anatomy

A brief anatomy survey can be easily completed at the same time as the fetal growth scan. Although by no means a comprehensive list, the findings or combination of findings listed in Table 65-3 should prompt the OB/GYN hospitalist to obtain a high-priority, detailed anatomy scan, a consultation with a perinatologist, or both, as these findings may radically alter the plan of care.