Ultrasound has permanently changed the image of perinatology, increasing expectations and improving outcomes in neonatal care. Ultrasound imaging is relatively inexpensive, safe, real time, and readily available in hospitals and clinics throughout the world. For almost 40 years sonography has progressed steadily with advances both in clinical application and equipment performance. It is truly an indispensable tool in obstetrics for the diagnosis and management of many diseases, encompassing three generations of women and millions of studies.39 Real-time ultrasound, in which image brightness varies with the intensity of returning signals (B-mode), is the standard method of fetal imaging (Figure 12-1). Typically, brief signal bursts are followed by relatively long receptive intervals; lower signal frequencies encounter less interference but reflect from fewer informative interfaces. Image quality varies with distance to the target, structure size, movement relative to the signal, and tissue transmission characteristics. Ideally, the transducer is in proximity to its target; transvaginal scans facilitate gynecologic examinations and early gestation and cervical studies. Suboptimal images are common with obesity, limited fluid interfaces, intervening structures, and poor positioning with respect to the sound beam. Diagnostic difficulty also increases when different materials have similar echo characteristics, as do blood, urine, and ascites. M-mode ultrasound is a direct representation of beam reflection by moving edges (e.g., in cardiovascular imaging). Interpretation requires hard-to-achieve, standardized stable views; fetal M-mode imaging has been de-emphasized because of improved B-mode resolution. M-mode is useful in assessing fetal heart arrhythmia, myocardial contractility, and pericardial effusion. M-mode “snapshots” also document cardiac activity and rate (Figure 12-2). Doppler ultrasound uses the frequency shift that occurs when the sound beam is reflected off moving objects to demonstrate the presence, velocity, and direction of blood flow. Pulsed sound waves are used to determine flow velocity from individual vessels (Figure 12-3). Direct volume calculations from narrow, tortuous fetal and uterine vessels are inaccurate. To compensate, prenatal Doppler findings, ideally obtained at beam angles less than 45 degrees for umbilical measurements and 15 degrees for middle cerebral velocities, are generally expressed as ratios. Color Doppler ultrasound semiquantitatively assigns direction to blood flow; by convention, warm colors denote movement toward the transducer, and saturation is keyed to velocity. Color Doppler illuminates cardiac, arterial, and venous structures (Figure 12-4 and Figure 12-5); any moving structure is potentially amenable to color Doppler detection. Color Doppler energy (power Doppler) is based on signal intensity; amplitude corresponds to blood cell motion. Power Doppler is effectively independent of angulation and is sensitive to very low flow; thus it is helpful for mapping vascular beds and for quickly spotting umbilical, pulmonary, middle cerebral, pericallosal, and other fetal vessels (Figure 12-6 and Figure 12-7). Three-dimensional ultrasound acquires and analyzes returning echoes along a third axis. Images are then manipulated electronically to show both surfaces and volumes from multiple perspectives, both as static and real-time (four-dimensional) views. Surface rendering of subtle facial details and of fetal small parts, multiplanar views, and stratified slices enhance detection of anomalies and partly overcome positional limitations of standard scans (Figure 12-8). Three-dimensional studies have facilitated volume calculations, analysis of complex spatial relationships, and have better explicated abnormal findings.12,13 Spatiotemporal correlation (database that manages both space and time information) of heart movement with color and power Doppler augments standard cardiac imaging (Figure 12-9). Skilled practitioners report significant gains in efficiency by off-line interpretation of acquired volumes in lieu of real-time scanning.12 Magnetic resonance imaging, usually performed after 20 weeks, provides high spatial resolution imaging without biological side effects.86 Maternal oblique positioning prevents inferior vena cava compression. Each image is obtained with an ultrafast sequence in less than 1 second. The large field of view, excellent soft tissue contrast, and multiple planes of construction make MRI an appealing imaging modality. It can supplement ultrasounds limited by maternal obesity or oligohydramnios.61 Usually MRI is used in the second and third trimesters to elucidate problems found on earlier ultrasound examinations. Fast T2-weighted sequences with single-shot fast spin-echo techniques are most commonly performed. Images are acquired in the axial, coronal, and sagittal planes to the fetus or orthogonal to the maternal pelvis. T1-weighted sequences detect recent hemorrhage and meconium-filled bowel. Diffusion imaging is useful for identifying ischemic injury to the brain. The use of gadolinium is relatively contraindicated but may be justified for assessment of placenta accreta or serious maternal disease.86 Claustrophobia can be difficult for some patients. Diagnostic ultrasound is a form of energy that has the potential for effects in tissues (bioeffects). The two most likely mechanisms are heating and cavitation.54,74 Although information regarding acoustic output for ultrasound examinations is limited, data collected during routine studies (first-trimester viability, nuchal translucency, and surveys including 3-dimensional/4-dimensional and growth) show that “gray-scale” B-mode ultrasound is associated with a negligible rise in temperature.77 Most pregnant women with access to modern obstetric care undergo sonographic examinations during the first and later trimesters. There has been no convincing evidence of harm in humans to date, although the endemic nature of early ultrasound exposure in obstetrics may require ingenuity in future study design.76 By definition, ultrasound is inaudible; moreover, it has not been identified as having oncogenic potential. Under nonclinical conditions, ultrasound energy may cause cell lysis, intracellular shearing, streaming effects, altered membrane permeability, and abnormal chromosome function. Small mammals experience smaller litters, impaired growth, and more anomalies after in utero exposures that do not always exceed comparable human levels. Current energy outputs greatly exceed those used in most of the original safety studies, with particular concern regarding effects on embryogenesis. Heat exposure triples with each change in modality: from 2- or 3D B-mode to M-mode, to color flow, before peaking during pulse Doppler. Harmful levels should not be reached during routine studies, but might occur during focal cranial pulse Doppler interrogation or in a febrile patient, without unusually long exposures.1 Mechanical disruption from cavitating gas bubbles is improbable in the fetus. Both temperature and disruption risks are now displayed on equipment (Figure 12-10); Thermal Index (TI), a ratio between transducer output and the energy needed to warm up tissue temperature by 1° C, with a desired value below 2, is also categorized by tissue type: TI soft tissues, TI cranial structures, and TI bone. Mechanical Index (MI) references pulse amplitude effects of compression and decompression, ideally maintained below 0.4 in fetal studies.1 Recommended levels may be exceeded when trying to achieve interpretable images during early gestation or in patients who are difficult to scan. For safety, only medically essential examinations should be performed; settings and duration should be the minimum required to achieve adequate views.6 Strong magnetic fields and radiofrequency waves are used in MRI with no known harmful effects, but large longitudinal studies are lacking. As with ultrasound, heat delivery to the fetus is a recognized hazard; in MRI, however, the maternal surface receives the greatest thermal exposure. Noise from the magnetic coils (up to 120 dB) is, in theory, capable of causing acute hearing damage; fortunately, maternal tissue attenuation decreases fetal intensities by 25%, to relatively safe levels. Direct magnetic bioeffects remain unproven; the FDA46 states that safety to the fetus “has not been established.”33 The education of physician sonologists encompasses visual recognition and interpretative tasks common to diagnostic imaging, but also demands mastery of specific mechanical skills (or, at least, an ability to assess the latter in sonographers). Ultrasound is an abbreviated module in general radiology and obstetric training with proportionate underrepresentation on certification examinations. Graduates may be obliged to acquire experience in the field to an undesirable degree, given the realities of medical practice and the serious consequences of error. Both ultrasound and MRI are rapidly evolving; clinically relevant frontiers are often explored by collaborators with pooled data, prerelease technology, and funding for staff and statistical analysis. Laudable advice from these investigators to confirm applicability or to undergo formal training before adopting new practices may be set aside by practitioners with fewer resources. Nuchal translucency width study, a deceptively simple sonographic method for aneuploidy screening, provides a cautionary example (Figure 12-11).80 Its orderly dissemination, including certification courses, ongoing audits, professional society, and laboratory coordination, is in contrast to the viral dissemination of many advances, yet reliability remains a concern. Ethical practitioners will be candid when informing patients of their ability to provide a requested service and assiduous in improving their skills. Professional judgment remains paramount in deciding how and when to incorporate new developments into clinical practice. Prenatal identification of fetal sex for the purpose of selective termination is available for serious X-linked disorders, but has been more widely applied to abort normal female fetuses because of a lower perceived value (Figure 12-12).90 No fully satisfactory response, whether acquiescence, nondisclosure, or noninspection, has been found for this abhorrent societal bias.40 Some businesses have recently started promoting nonmedical fetal ultrasound (also known as “keepsake” ultrasound), defined as using ultrasound to view, take a picture, or determine the sex of a fetus without a medical indication. Notwithstanding governmental and professional guidelines and warnings regarding ultrasound safety, the absence of proven harm, profitability, and popularity with prospective parents provided these businesses with an impetus for rapid expansion. A number of ethical issues are raised, including conflicts of interest for the commercial enterprise and the parents with respect to concerns about long-term effects. Current epidemiologic evidence is not synchronous with advancing ultrasound technology; a lack of evidence of harm is not the same as lack of harm. Fewer parents might request this exposure if fully educated; moreover, the fetus, ultimately the one at risk, cannot provide consent. Applying four major theories of ethics and principles (the precautionary principle, theories of consequentialism and impartiality, duty-based theory, and rights-based theories), it may be concluded that obstetric ultrasound practice is ethical only if the indication for its use is based on medical evidence, rendering “keepsake studies” ethically unjustifiable.55 Multiple gestations may result in a number of ethical dilemmas. Advances and regulation in assisted reproductive technology (ART) have decreased the incidence of multifetal pregnancies, but fetal reduction remains a painful choice for parents facing the prospect of extreme prematurity in higher order multiples. The management of twin-twin transfusion syndrome, anomalous co-twins, and discordant growth or distress far from term also necessitates choosing among unsatisfactory options. An excellent review of the psychosocial consequences and the ethical issues associated with selective termination of pregnancy has recently been published.53 Nondiagnostic studies, varying prognoses for a given diagnosis, and the inherent limitations of ultrasound and MRI studies lead to ethical issues in management and counseling. Physicians and patients alike may share unrealistic expectations for the predictive accuracy of targeted diagnoses.43 Anomalies and variants linked to Down syndrome and other syndromes (sonographic “markers”) discovered during routine studies present patients and caregivers with unanticipated, unwelcome choices, particularly if patients explicitly refused serum screening or direct genetic testing.39 Ideally, an informed consent discussion that addresses risks, benefits, consequences, and limitations of ultrasound or MRI studies should be provided to all patients before they choose to undergo such imaging.59,69 Given the irreversible nature of birth and abortion, ultimately prospective parents must judge for themselves their tolerance for uncertainty in diagnosis and for imperfection in their offspring. Genetic screening combines ultrasound study and biochemical testing to enhance the detection of chromosomal abnormalities, resulting in greater scrutiny of younger patients and less frequent invasive testing for women older than 35 years5 (see Chapter 11). Presently noninvasive screening for fetal aneuploidy (trisomies 13, 18, 21) is offered to all pregnant women regardless of maternal age. Invasive prenatal diagnosis is offered to women with results above a predetermined value on noninvasive screening, to those with risks based on personal, medical or family history, and to patients requesting the procedure regardless of risk status, after appropriate counseling. Common noninvasive screening options combine maternal age with one of the following: (1) first-trimester screening (nuchal translucency and maternal serum biochemical markers), (2) second-trimester serum screening (maternal serum biochemical markers), or (3) two-step integrated screening, which includes first- and second-trimester serum screening with or without nuchal translucency (integrated prenatal screen, serum integrated prenatal screening, contingent and sequential screening are variations). Different algorithms noticeably affect sensitivity, specificity, and predictive values; cost or convenience may factor in choosing a strategy.30 Ultrasound examination of the conceptus between 11 and 13 weeks’ gestation provides important information about aneuploidy, genetic syndromes and congenital anomalies. Many fetal structural anomalies can be detected at this gestational age. Nuchal translucency measurement in conjunction with maternal serum markers (free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A), has been shown to be a highly effective first-trimester method, further improved by the addition of other ultrasound markers. Cell-free DNA and PCR-based testing is evolving rapidly; it is premature to predict to what extent and how quickly these commercially available tests will replace or supplement current screening protocols. At least one third of fetuses with Down syndrome will have anomalies, notably endocardial cushion defects (Figure 12-13), duodenal atresia (Figure 12-14), and less detectably, small atrioseptal and ventriculoseptal cardiac defects (Figure 12-15). Two thirds may have sonographic markers, not limited to: increased nuchal lucency, hypoplastic nasal bones, and abnormal cardiovascular Doppler patterns in the first trimester; second-trimester findings of thickened nuchal fold (Figure 12-16) and nasal hypoplasia; ventriculomegaly; choroid cysts (Figure 12-17); hypoplasia of the fifth digit; decreased long bone ratios; enhanced echogenicity of papillary muscles and bowel; and renal pyelectasis (Figure 12-18). Screen components may be affected by ethnicity, habitus, diet, and fetal sex, as well as by device and operator-dependent detection rates. The permutations may stymie counselors attempting to provide (and patients trying to grasp) basic descriptions of risks and benefits. Recently described first-trimester evaluation of the posterior brain (intracranial translucency) provides an additional screening tool for the presence of open neural tube defects. Doppler documentation of tricuspid regurgitation and increased ductus venosus resistance in the first trimester seems a promising addition to early screening protocols.56 Doppler measurement of the pulsatility index in the uterine arteries, in conjunction with maternal history, examination, and serum biochemical testing may aid in predicting preeclampsia.81 Ultrasound is essential for timing and guiding oocyte retrieval and helpful in embryo transfer; its role in judging endometrial receptivity is less clear. Saline ultrasound studies prior to fertility treatments routinely complement or replace hysterosalpingograms in assessment of uteri and adnexa.2 Use of MRI may play an expanded role in structural and functional evaluation in the future. Assisted reproductive technology results in more twins and higher-order multiples; early ultrasound study of embryos, amnionicity, and chorionicity is essential to subsequent management. With the increasing popularity of ART, obstetricians and radiologists are more likely to encounter associated complications, especially in an emergency setting. These complications include ovarian hyperstimulation or torsion, ectopic or heterotopic pregnancy, and pregnancies of unknown location or viability. Ovarian hyperstimulation syndrome may occur following ovulation induction or ovarian stimulation and is characterized by bilateral ovarian enlargement, by multiple cysts, and third-spacing of fluids. Additional clinical findings may range from gastrointestinal discomfort to life-threatening renal failure and coagulopathy (Figure 12-19). Enlarged, cystic and hyperstimulated ovaries are at risk for torsion, but clinical symptoms are often nonspecific. Ovarian torsion should be excluded in any woman undergoing ART who presents with severe abdominal pain. The most consistent imaging finding is asymmetric enlargement of the ovary twisted on its pedicle, although loss of Doppler vascular flow is considered more specific. Ectopic pregnancy resulting from ART has a relatively increased frequency of rarer and more lethal forms, including interstitial and cervical locations. Heterotopic pregnancies, simultaneous intrauterine and ectopic implantations, have a higher incidence in ART patients.3 Traditionally, first-trimester transvaginal scans exclude ectopic implantation and confirm viability by demonstrating an intrauterine asymmetric or “double sac,” gestational sac with yolk sac or embryo, preferably with cardiac activity; identifiable embryos in the salpinges are uncommon. Heterotopic pregnancy occurs in less than 0.1% of patients, leading to the pragmatic approach that finding an intrauterine pregnancy usually excludes an ectopic one.11 A “double sac” is usually seen transvaginally at levels of 1000 to 1500 international units of human chorionic gonadotropin before Embryonic heart rates, slower at initiation, increase to more than 160 beats per minute (bpm) by the ninth week and then decline slightly through the 13th week. Persistent rates below 100 bpm are often linked to abortion, aneuploidy, and anomalies.35 When a slow embryonic heart rate is detected between 6 and 7 weeks, likelihood of first-trimester demise remains elevated (approximately 25%) even if the heart rate normalizes. In such pregnancies, a follow-up scan is warranted. Prior to 6.1 weeks, an embryonic heart rate less than 100 bpm is not necessarily a poor prognostic indicator. The likelihood of survival into the second trimester is also significantly higher when there is concordance between biometrically calculated gestational age and menstrual dating than if there is discordance.7 Embryonic anatomic surveys are limited and necessarily provisional; however, early fetal period scans diagnose a number of entities accurately (Figure 12-20). Midtrimester confirmation continues to be prudent for the majority of first-trimester findings. Later studies retain advantages with respect to the natural history of many anomalies and for visualization of heart, spine, and other problematic structures. Multiple pregnancy (see Chapter 22) accounts for about 3% of all pregnancies. With an increase in the number of fetuses, scans become more complex, time consuming, and error prone; additionally, determination of zygosity is essential. The management of anomalous, discordant, or moribund co-twins differs significantly based on chorionicity (Figure 12-21). Monochorionic twins occur with a relatively constant frequency (1 : 250 pregnancies) unlike dichorionic twinning that may be influenced by race, heredity, maternal age, parity, and medically assisted procreation. Ultrasound assignment of chorionicity is most accurate for different-sexed dizygotic twins, but approaches this accuracy in gender concordant pairs by evaluating sac appearance in early gestation. Successful identification may occur throughout gestation by examining the dividing membranes at their placental origin. Dichorionic diamniotic twins are usually (95%) dizygotic, with independent risks for anomalies and placental malfunction. Monochorionic pairs are predictably monozygotic; attrition rates exceed 30% from early abortion, anomalies, and prematurity. Matched and isolated anomalies are both more common in monochorionic gestations; because of shared vasculature, loss of a co-twin may kill its sibling outright or produce severe neurologic damage in up to one third of survivors.57 One of the most powerful applications of prenatal ultrasound is using biometrics to establish or confirm gestational age.21 Through 22 weeks of gestation, most genetically normal individuals cluster closely on nomographic curves.27 Sonographic measurements of fetal ultrasound parameters are the basis for accurate determination of gestational age and detection of fetal growth abnormalities. It has been shown that a fixed error of about 8% (plus or minus) can be anticipated when determining gestational age by ultrasound, consistent with the observation that the earlier gestational age is determined, the lower the margin of error in days.41 If accelerated or restricted growth supervenes, however, most biometric dating is compromised accordingly. Selection of the most useful single parameter depends on the timing and purpose of measurement and is influenced by specific limitations. More than 50 fetal structures have now been measured by ultrasound, with results expressed as a function of gestational age. Commercial equipment has a variety of preinstalled nomograms, biometry-based dating, and weight estimation algorithms. Crown-rump length is considered the best parameter for early dating of pregnancy. Biparietal diameter (BPD), obtainable after parietal bone calcification in week 12, maintains the closest correlation with gestational age in the second trimester. Biparietal diameter is measured from the outer to the inner table of the skull, perpendicular to the parietal bones and central falx cerebri. The proper plane contains the cavum septum pellucidum, thalami, third ventricle, and tentorial hiatus, within the bony table of a complete head circumference (Figure 12-22). In cases of variation in the shape of the skull, a head circumference (HC) measurement obtained in the same plane may be an effective alternative. Sonographic estimation of HC is associated with significant underestimation compared with the actual postnatal HC. This measurement error may have important clinical implications and should be taken into account in the interpretation of sonographically measured HC.62 Biparietal diameter and HC are relatively spared in nutritional and perfusion disorders of growth, although cranial measurements may be distorted by compression. The abdominal circumference, measured along the outer margin of the abdominal skin line at the level of the gastric bubble and the intrahepatic portion of the umbilical vein at the bifurcation of the portal veins, is the best single predictor of growth aberrations but is less helpful for dating (Figure 12-23). Femoral length is the fourth measurement commonly included in biometric formulae. Extraocular and transcerebellar diameters, and humeral and pedal lengths are common additional dating parameters. Special curves are available for non-Caucasian ethnicity or multiple gestations. Serial measurements over time are the best way to judge fetal growth. The sac and embryo grow perceptibly each day; by the second trimester, intervals of 2 or 3 weeks between studies are more reliable. Problems arise when attempting simultaneously to assign age and weight percentile without reliable dating. The abdominal circumference is a better indicator of decreased perfusion or increased glycogen storage than the cranial measures; ratios of abdominal circumference to the head and femur amplify the differences but have poor sensitivity and specificity. Strategies to identify small-for-date fetuses have included risk panels, Doppler ratios, amniotic fluid volume, placental scores, and biometry-based weights; none have had completely satisfactory results, although outcomes appear to have improved.72 The placenta functions to nourish and protect the fetus. Imaging of the placenta can have a profound impact on patient management, owing to the morbidity and mortality associated with various placental conditions. Placental conditions affecting the mother and fetus include molar pregnancies, placental hematoma, abruption, previa, accreta, vasa previa, choriocarcinoma, and retained products of conception. Ultrasonography remains the definitive modality in diagnosing most of these conditions, with magnetic resonance imaging remaining an adjunctive measure. Computed tomography is occasionally used in cases of trauma and tumor staging.63 Placental location can be confidently established by transvaginal ultrasound study. Low-lying placentation persists as placenta previa in only 1% to 2% of patients at term (Figure 12-24). Using color Doppler, fetal vessels can be seen near the cervix, facilitating the diagnosis of funic presentation and vasa previa (Figure 12-25). Ultrasound is also helpful in finding placental accretion, the absence of a normal cleavage plane between decidua and placental vessels. Accretion, deeper penetration of the vessels into the myometrium (incretion), and serosal penetration (percreta) were previously diagnosed only when failed attempts to deliver the placenta were followed by massive hemorrhage. Placental accretion is more likely in women with placenta previa and a history of cesarean section,70 after myomectomy or curettage, and with high parity.38 The frequency of accretion has increased more than 10-fold in the past 20 years to 1 in 2500, echoing rising cesarean rates.36,87 Sonographic criteria for placenta accreta were developed using 2D gray-scale transabdominal and transvaginal ultrasonography. The sonographic findings tested were loss/irregularity of the echo-free “clear space” between the uterus and the placenta, thinning or interruption of the hyperechoic interface between the uterine serosa and the bladder wall, the presence of turbulent placental lacunae with high-velocity flow, as well as hypervascularity of the uterine serosa–bladder wall interface and irregular intraplacental vascularization by transabdominal 3D power Doppler.78 Diagnostic ultrasound findings of indistinct placental margins, attenuated myometrium, and large turbulent placental vessels (mainly veins) have sensitivity and specificity for accretion in the range of 85%.28 The presence or absence of the listed findings has been shown to be very helpful in diagnosing placental accretion and in differentiation of placenta accreta from percreta.20 Antepartum diagnosis permits planning for anesthesia, transfusion, balloon tamponade, arterial embolization, or scheduled preterm cesarean-hysterectomy prior to onset of labor. (Figure 12-26). Magnetic resonance imaging can be helpful when there is a posterior placenta, after myomectomy, if percreta is suspected and when ultrasound findings are ambiguous.9,37 The diagnosis of placental abruption remains a clinical one. Ultrasound diagnosis is reported to have only 24% sensitivity in the third trimester, although specificity may reach 88%.42 The role of imaging in this disorder is to exclude placenta previa, an equally common source of severe third-trimester bleeding. Subchorionic hematomas are often noted on transvaginal scans early in gestation; symptomaticity, size, and persistence have been linked to poorer outcomes.83 Later abruptions are more difficult to visualize; acute bleeding is isoechoic with placenta and can be mistaken for placentomegaly. Hypoechoic fluid collections and hyperechoic infarcted areas appear in more chronic presentations. Grading placental appearance to detect disturbed growth or maturation is of limited benefit. Persistent immaturity is linked to hydrops fetalis, although not as often as increased echogenicity and thickening. Precociously mature placentas may presage growth restriction (Figure 12-27). Amniotic fluid volume is initially determined by secretions from the amnion; however, by the 16th week of pregnancy, fetal renal production accounts for the majority (see Chapter 25). Malformations of the esophagus and upper gastrointestinal tract, inhibited fetal swallowing, aneuploidy, intermittent renal obstruction, maternal diabetes, twin-twin transfusion syndrome, some dwarfisms, and fetal hydrops are associated with marked polyhydramnios. Severe growth restriction with polyhydramnios carries a poor prognosis. Polyhydramnios is idiopathic in almost half of cases; consequences include prematurity, malposition, abruption, preeclampsia, and puerperal hemorrhage. Abnormal volumes are subjectively apparent to experienced examiners but quantification remains a research issue. Maximum pocket or summed depths across quadrants conveniently serve as proxies for volume in clinical settings (Figure 12-28). Oligohydramnios may occur after membrane rupture; after fetal renal compensation for placental hypoperfusion; from functional or obstructive urogenital anomalies; with maternal dehydration; and following exposure to some medications, including indomethacin and angiotensin-converting-enzyme inhibitors. Amniotic fluid measurements may be combined with non-stress and fetal biophysical testing (see Fetal Well-Being Assessment in this chapter) to provide reassurance of fetal well-being. Mechanisms of amniotic fluid dynamics, the role of fluid assessment in clinical care, and the appropriate treatment for abnormalities remain poorly understood.73
Perinatal Imaging
Fetal Imaging Techniques
Bioeffects and Safety
-week intrauterine pregnancy, showing a small yolk sac (thin arrows) within the gestational sac (calipers) and echogenic surrounding decidual reaction (thick arrows). The embryo is not yet visible. Note MI and TI notations at top center.
Ethical Considerations
Applications of Ultrasound
Genetic Screening
Assisted Reproduction
First-Trimester Studies
menstrual weeks (see Figure 12-10), but hormonal levels and thresholds for visualization are variable. Finding a yolk sac or embryo confirms an intrauterine site. The gestational sac diameter grows about 1 mm daily in early pregnancy, and the embryonic disc is usually visible transvaginally once sac diameters exceed 15 mm. Embryonic lengths also increase by 1 mm daily; transvaginal documentation of cardiac activity is customary by the end of the sixth week (4 mm embryonic length).50
Multiple Gestations
Pregnancy Evaluation
Fetal Growth
Placental Abnormalities
Placental Location
Amniotic Fluid Volume
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Perinatal Imaging
