The impact of sonography on the practice of obstetrics has been profound. A carefully performed ultrasound examination can reveal vital information about fetal anatomy, physiology, and well-being. Recent technological achievements have resulted in dramatic improvements in resolution and image display. Doppler applications have expanded, and 3-dimensional ultrasound imaging has continued to evolve. The reader is referred to Williams Obstetrics, 23rd ed., Chapter 16, for discussions of magnetic resonance imaging of the fetus and fetal therapy for selected abnormalities.
Sonography should be performed only with a valid medical indication and with the lowest possible exposure setting to gain necessary information—the ALARA principle, As Low As Reasonably Achievable. Duplex Doppler coupled with realtime imaging requires monitoring of the thermal index. Microbubble ultrasound contrast agents are not used in pregnancy because they might raise the mechanical index. The American Institute of Ultrasound in Medicine recommends that fetal sonography be performed only by professionals trained to recognize medically important conditions such as fetal anomalies, artifacts that may mimic pathology, and techniques to avoid ultrasound exposure beyond what is considered safe for the fetus.
Indications for performing sonography in the first 14 weeks are listed in Table 9-1. The components listed in Table 9-2 should be assessed. With transvaginal scanning, the gestational sac is reliably seen in the uterus by 5 weeks, and fetal echoes and cardiac activity by 6 weeks. Cardiac motion is typically observed when the embryo has reached 5 mm in length.
TABLE 9-1. Some Indications for First-Trimester Ultrasound Examination
TABLE 9-2. Components of Standard Ultrasound Examination by Trimester
This is the maximum thickness of the subcutaneous translucent area behind the skin and soft tissue overlying the fetal spine at the back of the neck (see Figure 9-1). It is measured in the sagittal plane between 11 and 14 weeks using precise criteria (see Table 9-3). As discussed in Chapter 3, nuchal translucency measurement, combined with maternal serum chorionic gonadotropin and pregnancy-associated plasma protein A assessment, has gained widespread use for first-trimester aneuploidy screening.
FIGURE 9-1 The nuchal translucency (NT) measurement is the maximum thickness of the subcutaneous translucent area between the skin and soft tissue overlying the fetal spine at the back of the neck. Calipers are placed on the inner borders of the nuchal space, at its widest position, perpendicular to the long axis of the fetus. In this normal fetus at 12 weeks’ gestation, the measurement is 2.0 mm. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010. Used with permission from Dr. Robyn Horsager.)
TABLE 9-3. Guidelines for Nuchal Translucency (NT) Measurement
SECOND- AND THIRD-TRIMESTER ULTRASOUND
Some of the many indications for second- and third-trimester sonography are listed in Table 9-4. These examinations can be categorized as standard, specialized, or limited. Table 9-2 lists the standardexamination components, one of which is a survey of fetal anatomy. Elements of the fetal anatomy survey are listed in Table 9-5. Fetal anatomy may be adequately assessed after 18 weeks. If a complete survey of fetal anatomy cannot be obtained—for example, due to oligohydramnios, fetal position, or maternal obesity—it should be noted in the report.
TABLE 9-4. Some Indications for Second- or Third-Trimester Ultrasound Examination
TABLE 9-5. Minimal Elements of a Standard Examination of Fetal Anatomy
Specialized examinations are performed and interpreted by an expert sonologist who determines the examination components on a case-by-case basis. One type of specialized study is a targeted examination, a detailed anatomical survey performed when an anomaly is suspected on the basis of history, maternal serum screening test abnormality, or abnormal findings from a standard examination. Other specialized examinations include fetal echocardiography, Doppler evaluation, biophysical profile, or additional biometric studies.
A third type of procedure is the limited examination, performed when a specific question requires investigation, such as determination of amnionic fluid volume or fetal presentation. In most cases, limited examinations are appropriate only when a prior complete examination is on record.
Various formulas and nomograms allow accurate assessment of gestational age and describe normal growth of fetal structures. In the first trimester, equipment software computes the estimated gestational age from the crown-rump length. In the second and third trimesters, gestational age and fetal weight are estimated using measurements of the biparietal diameter, head and abdominal circumference, and femur length. Estimates are typically most accurate when multiple parameters are used, with nomograms derived from fetuses of similar ethnic or racial background living at similar altitude. Even the best models may over- or underestimate fetal weight by as much as 15 percent.
The variability of gestational age estimation increases with advancing pregnancy. Crown-rump length in the first trimester can be used to accurately determine gestational age to within 3 to 5 days (see Appendix B, Table B-1). Between 14 and 26 weeks, the biparietal diameter (BPD) and femur length (FL) are most accurate for estimating gestational age. The BPD has a variation of 7 to 10 days, and the FL has a variation of 7 to 11 days. The head circumference (HC) is more reliable than the BPD if the head shape is flattened—dolichocephaly, or rounded—brachycephaly. The abdominal circumference (AC) is the parameter most affected by fetal growth and has the widest variation, up to 2 to 3 weeks. A nomogram showing the average gestational age associated with each of these parameters is found in Appendix B, Table B-2. By the third trimester, individual measurements are least accurate, and estimates are improved by averaging the four parameters. Sonography performed to evaluate fetal growth should typically be performed at least 2 to 4 weeks apart.
Determination of the amount of amnionic fluid is an important method of fetal assessment. The most widely used measurement is the amnionic fluid index, calculated by adding the depth in centimeters of the largest vertical pocket in each of four equal uterine quadrants. Normal values are shown in the Appendix B, Table B-5. In most normal pregnancies, the AFI ranges between 8 and 24 cm. Another method measures the largest vertical pocket of amnionic fluid. The normal range is 2 to 8 cm—values less than 2 cm signify oligohydramnios, whereas those greater than 8 cm define hydramnios. Abnormalities associated with too much or too little amnionic fluid are presented in Chapters 10 and 11.
NORMAL AND ABNORMAL FETAL ANTOMY
An important goal of sonographic evaluation is to categorize fetal components as anatomically normal or abnormal. Deviations from normal require that a specialized examination be performed. In the following discussion, only a few of the literally hundreds of fetal anomalies are described.
Central Nervous System
Three transverse (axial) views of the fetal brain are imaged: (1) the transthalamic view is used to measure BPD and HC and includes the thalami and cavum septum pellucidum; (2) the transventricular viewincludes the atria of the lateral ventricles that contain the echogenic choroid plexus; and (3) the transcerebellar view of the fetal posterior fossa includes the cerebellum and cisterna magna. Between 15 and 22 weeks, the cerebellar diameter in millimeters is roughly equivalent to the gestational age in weeks.
Neural-tube defects occur in 1.6 per 1000 live births in the United States and result from incomplete closure of the neural tube by the embryonic age of 26 to 28 days. There are 3 main types. Anencephaly is a lethal defect characterized by absence of the brain and cranium above the base of the skull and orbits (see Figure 9-2). It can be diagnosed as early as the first trimester, and hydramnios commonly develops in the third trimester. Cephalocele is a herniation of meninges and brain tissue through a cranial defect, typically an occipital midline defect. Associated hydrocephalus and microcephaly are common, and there is a high incidence of mental impairment among surviving infants. Spina bifida is an opening in the vertebrae through which a meningeal sac may protrude, forming a meningocele. If the sac contains neural elements, as it does in 90 percent of cases, the anomaly is called a meningomyelocele (see Figure 9-3). Classically, fetuses with spina bifida have one or more of the following sonographic cranial findings: scalloping of the frontal bones—the so-called “lemon sign” (see Figure 3-1), bowing of the cerebellum with effacement of the cisterna magna—the “banana sign” (see Figure 3-2), small biparietal diameter, and ventriculomegaly. Prenatal screening for neural-tube defects is reviewed in Chapter 3.
FIGURE 9-2 Anencephaly. This sagittal image shows the absence of forebrain and cranium above the skull base and orbit. The long white arrow points to the fetal orbit, and the short white arrow indicates the nose. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 9-3 Sagittal (A) and transverse (B) views of the spine in a fetus with a large lumbosacral meningomyelocele (arrow). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Enlargement of the cerebral ventricles is a general marker of abnormal brain development. Ventriculomegaly may be caused by a wide variety of genetic and environmental insults, and prognosis is determined both by etiology and rate of progression. Mild ventriculomegaly is diagnosed when the atrial width measures 10 to 15 mm (see Figure 9-4), and overt or severe ventriculomegaly when it exceeds 15 mm. Even when isolated, mild ventriculomegaly has been associated with developmental delay in up to a third of affected infants. As the atrial measurement increases further, so does the likelihood of abnormal outcome. Initial evaluation includes a thorough survey of fetal anatomy, fetal karyotype, and testing for congenital viral infections such as cytomegalovirus and toxoplasmosis.
FIGURE 9-4 The atria appear unusually prominent in this fetus with mild ventriculomegaly (caliper measurement 12 mm).
With this abnormality, the prosencephalon fails to divide completely into two separate cerebral hemispheres and diencephalic structures. In the most severe form, alobar holoprosencephaly, a single monoventricle surrounds the fused thalami (see Figure 9-5). There may be associated midline facial anomalies, including hypotelorism, cyclopia, arhinia, proboscis, or median cleft. The birth prevalence is 1 in 10,000 to 15,000, and approximately half of all cases have a chromosomal abnormality, particularly trisomy 13. Fetal karyotyping should be offered when this anomaly is identified.
FIGURE 9-5 In this 14-week fetus with a lobar holoprosencephaly, the thalami are fused (FT) and surrounded by a monoventricle (V). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
This abnormality of the posterior fossa is characterized by agenesis of the cerebellar vermis, enlargement of the cisterna magna, and elevation of the tentorium. The birth prevalence is approximately 1 per 12,000. Dandy-Walker malformation is associated with a large number of genetic and sporadic syndromes, aneuploi-dies, congenital viral infections, and some teratogens, all of which greatly affect the prognosis. Thus, the initial evaluation mirrors that for ventriculomegaly. Even when vermian agenesis appears to be partial and relatively subtle, there is a high incidence of associated anomalies, and the prognosis is typically poor.
This is a malformation of the lymphatic system in which fluid-filled sacs extend from the posterior neck. Cystic hygromas typically develop as part of a lymphatic obstruction sequence, in which lymph from the head fails to drain into the jugular vein and collects instead in jugular lymphatic sacs. The enlarged thoracic duct can impinge on the developing heart. Approximately 60 to 70 percent are associated with fetal aneuploidy. Of fetuses with cystic hygroma diagnosed in the second trimester, approximately 75 of aneuploidy cases are 45X—Turner syndrome. When cystic hygromas are diagnosed in the first trimester, the most common aneuploidy is trisomy 21. Cystic hygromas also may be an isolated finding or part of a genetic syndrome. Large, multiseptated lesions rarely resolve, often lead to hydrops fetalis, and carry a poor prognosis.
The lungs are best visualized after 20 to 25 weeks of gestation and appear as homogeneous structures surrounding the heart, occupying approximately two-thirds the area of the chest in the four-chamber view. A variety of thoracic malformations, including cystic adenomatoid malformation, extralobar pulmonary sequestration, and bronchogenic cysts may be seen sonographically as cystic or solid space-occupying lesions.
The incidence of congenital diaphragmatic hernia is approximately 1 per 4000 births. In 90 percent of cases, the diaphragmatic defect is left sided and posterior, and the heart may be pushed to the middle or right side of the thorax by the stomach or bowel. With improved technology, visualization of the liver within the thorax is increasingly common. Almost half of cases are associated with other major anomalies or aneuploidy. A thorough evaluation of all fetal structures should be performed, and amniocentesis for fetal karyotype should be offered.
Cardiac malformations are the most common congenital anomalies, with an incidence of approximately 8 per 1000 live births. Nearly 30 to 40 percent of cardiac defects diagnosed prenatally are associated with chromosomal abnormalities. Fortunately, at least 50 percent of aneuploid fetuses also have extracardiac anomalies that are identifiable sonographically. The most frequently encountered aneuploi-dies are Down syndrome, trisomies 18 and 13, and Turner syndrome (45, X).
A standard assessment includes the four-chamber view (see Figure 9-6), and if technically feasible, evaluation of the left and right ventricular outflow tracts (see Figure 9-7). The four-chamber view allows evaluation of cardiac size, position in the chest and axis, atria and ventricles, foramen ovale, atrial septum primum, interventricular septum, and atrioventricular valves. The two atria and ventricles should be similar in size, with the apex of the heart forming a 45-degree angle with the left anterior chest wall. A specialized examination with fetal echocardiography is typically performed if there are any of the following: abnormality noted in the four-chamber or outflow tract views, arrhythmia, presence of extracardiac anomaly that confers increased risk, known genetic syndrome that may include a cardiac defect, increased nuchal translucency in the first trimester in a fetus with normal karyotype, insulin-treated diabetes prior to pregnancy, family history of congenital heart defect, or teratogen exposure.
FIGURE 9-6 Four chamber view of the fetal heart, showing the location of the left and right atria (LA, RA), left and right ventricles (LV, RV), foramen ovale (FO), and descending thoracic aorta (A). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 9-7 Views of the left and right ventricular outflow tracts. A. The left ventricular outflow tract demonstrates the continuity of the interventricular septum (IVS) and mitral valve (M) with the walls of the aorta (Ao). B. The right ventricular outflow tract shows the normal orientation of the aorta (Ao) and pulmonary artery (PA). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
The integrity of the abdominal wall at the level of the cord insertion is assessed during the standard examination.
This is a full-thickness abdominal wall defect typically located to the right of the umbilical cord insertion, and bowel herniates into the amnionic cavity (see Figure 9-8). The incidence is 1 per 2000 to 5000 pregnancies, and it is the one major anomaly more common in infants of younger mothers. This anomaly is not associated with an increased risk for aneuploidy and usually has a survival rate of approximately 90 percent. Associated bowel abnormalities such as jejunal atresia are found in 15 to 30 percent of cases and are believed to be a result of vascular damage or mechanical trauma.
FIGURE 9-8 Transverse view of fetal abdomen. In this fetus with gastroschisis, extruded bowel loops are floating in the amnionic fluid to the right of the normal umbilical cord insertion site (arrow). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
This anomaly complicates approximately 1 per 5000 pregnancies. It occurs when the lateral ectomesodermal folds fail to meet in the midline, leaving abdominal contents covered only by a two-layered sac of amnion and peritoneum (see Figure 9-9). The umbilical cord inserts into the apex of the sac. In over half of cases, an omphalocele is associated with other major anomalies or aneuploidy. It is also a component of syndromes such as Beckwith–Wiedemann, cloacal exstrophy, and pentalogy of Cantrell. Identification of an omphalocele mandates a complete fetal evaluation, and fetal karyotype is recommended.
FIGURE 9-9 Transverse of the abdomen showing an omphalocele as a large abdominal wall defect with exteriorized liver covered by a thin membrane. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
The stomach is visible in 98 percent of fetuses after 14 weeks, and the liver, spleen, gallbladder, and bowel can be identified in many second- and third-trimester fetuses. Nonvisualization of the stomach within the abdomen is associated with a number of abnormalities, such as esophageal atresia, diaphragmatic hernia, abdominal wall defects, and neurological abnormalities that inhibit fetal swallowing.
Most atresias are characterized by obstruction with proximal bowel dilatation. In general, the more proximal the obstruction, the more likely it is to be associated with hydramnios. Esophageal atresia may be suspected when the stomach cannot be visualized and hydramnios is present. That said, in up to 90 percent of cases of espophageal atresia, a concomitant tracheoesophageal fistula allows fluid to enter the stomach—thus prenatal detection is problematic. Associated anomalies are common, particularly cardiac malformations, and aneuploidy complicates 20 percent. Duodenal atresia may be detected prenatally by the demonstration of the double-bubble sign, which represents distention of the stomach and first part of the duodenum (see Figure 9-10). Demonstrating continuity between the stomach and the proximal duodenum will differentiate duodenal atresia from other causes of an abdominal cyst. Approximately 30 percent of cases diagnosed antenatally have trisomy 21, and more than half have other anomalies. Obstructions in the lower small bowel usually result in multiple dilated loops that may have increased peristaltic activity.
FIGURE 9-10 Double-bubble sign of duodenal atresia is seen on this axial abdominal image of the fetus. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Kidneys and Urinary Tract
The fetal kidneys are routinely visualized by 18 weeks. The placenta and membranes produce amnionic fluid early in pregnancy, but after 18 weeks, most of the fluid is produced by the kidneys. Unexplained oligohydramnios should prompt an evaluation for a urinary tract abnormality.
One or both kidneys are congenitally absent in 1 in 4000 births. The kidney is not visible, and the adrenal typically enlarges to fill the renal fossa—aptly termed the lying down adrenal sign. If renal agenesis is bilateral, no urine is produced, and the resulting severe oligohydramnios leads to pulmonary hypoplasia, limb contractures, a distinctive compressed face, and ultimately death. When this combination of abnormalities results from renal agenesis, it is called Potter syndrome, after Dr. Edith Potter who described it in 1946.
Polycystic Kidney Disease
Of the hereditary polycystic diseases, only the infantile form of autosomal recessive polycystic kidney disease may be reliably diagnosed antenatally. It is characterized by abnormally large kidneys that fill the fetal abdomen and appear to have a solid, ground-glass texture. The abdominal circumference is enlarged, and there is severe oligohydramnios. The cystic changes can only be identified microscopically.
Multicystic Dysplastic Kidneys
Multicystic renal dysplasia results from obstruction or atresia at the level of the renal pelvis or proximal ureter prior to 10 weeks. There is abnormally dense renal parenchyma with multiple cysts of varying size that do not communicate with the renal pelvis (see Figure 9-11). The prognosis is generally good if findings are unilateral and the amnionic fluid volume is normal.
FIGURE 9-11 Coronal view of the fetal abdomen and lower thorax displays multiple cysts of varying sizes, which do not communicate in the retroperitoneal region of this fetus with multicystic dysplastic kidneys. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Ureteropelvic Junction Obstruction
This condition is the most common cause of neonatal hydronephrosis and affects males twice as often as females. The actual obstruction is generally functional rather than anatomical, and is bilateral in one-third of cases (see Figure 9-12). Unilateral obstruction is associated with an increased risk of anomalies in the other kidney. A commonly used upper limit for the normal renal pelvis diameter is 4 mm before 20 weeks, and if this limit is exceeded, sonography is performed again at approximately 34 weeks. If the renal pelvis diameter exceeds 7 mm at 34 weeks, evaluation in the neonatal period should be considered.
FIGURE 9-12 Transverse view of the kidneys demonstrating hydronephrosis (calipers) in a fetus with ureteropelvic junction obstruction. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Collecting System Duplication
This is the most common genitourinary anomaly and is found in up to 4 percent of the population. The characteristic obstruction of the upper pole is evident as pyelectasis, often associated with a dilated ureter and an ectopic ureterocele within the bladder. Reflux of the lower pole moiety is common.
Bladder Outlet Obstruction
This distal obstruction of the urinary tract is more common in male fetuses, and the most common etiology is posterior urethral valves. Characteristically, there is dilatation of the bladder and proximal urethra—with the urethra resembling a keyhole—along with thickening of the bladder wall. Oligohydramnios portends a poor prognosis because of pulmonary hypoplasia. Unfortunately, the outcome is not uniformly good even with a normal amount of fluid. Prenatal diagnosis allows some affected fetuses to benefit from early intervention postnatally or even consideration of in-utero therapy.
The goal of 3-D imaging is to obtain a volume and then render it to enhance real-time 2-dimensional findings. Because of the obvious appeal of a 3-D portrait of the fetal face, surface rendering is the most popular technique (see Figure 9-13). And for selected fetal anomalies, 3-D may provide useful information. The ability to reformat images in any plane may facilitate evaluation of the corpus callosum and fetal palate, and improvements in postprocessing algorithms and techniques may improve visualization of cardiac anatomy. Limitations of 3-D sonography include longer time to complete the study due to image processing, crowding by adjacent structures that may obscure the captured image, and challenges posed by the need to store and manipulate large amounts of data. Importantly, comparisons of 3-D with conventional 2-D sonography for the diagnosis of most congenital anomalies have not demonstrated an improvement in overall detection. Thus, the precise utility of this exciting technology has yet to be fully determined.
FIGURE 9-13 Surface rendered 3-dimensional image of the fetal face at 33 weeks’ gestation. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Doppler velocimetry is a noninvasive way to assess blood flow by characterizing downstream impedance. The Doppler principle states that when sound waves strike a moving target, the frequency of the sound waves reflected back is shifted proportionate to the velocity and direction of the moving target (see Figure 9-14). In obstetrics, Doppler may be used to determine the volume and rate of blood flow through maternal and fetal vessels. The sound source is the ultrasound transducer, the moving target is the column of red blood cells flowing through the circulation, and the reflected sound waves are observed by the ultrasound transducer. Figure 9-15 demonstrates normal Doppler waveforms from several maternal and fetal vessels. An important source of error when calculating flow or velocity is the angle between sound waves from the transducer and flow with the vessel—termed the angle of insonation and abbreviated as θ. Because cosine θ is a component of the equation, measurement error becomes large when the angle of insonation is not close to zero. For this reason, ratios are used to compare different waveform components, allowing cosine θ to cancel out of the equation. Figure 9-16 is a schematic of the Doppler waveform and describes the three ratios commonly used.
FIGURE 9-14 Doppler equation: Ultrasound emanating from the transducer with initial frequency fo strikes blood moving at velocity v. Reflected frequency fd is dependent on angle θ; between beam of sound and vessel. (Copel JA, Grannum PA, Hobbins JC, et al: Doppler ultrasound in obstetrics. Williams Obstetrics. 17th ed. (Suppl 16), Norwalk, CT, Appleton and Lange; 1988.)
FIGURE 9-15 Doppler waveforms from normal pregnancy. Shown clockwise are normal waveforms from the maternal arcuate, uterine, and external iliac arteries, and from the fetal umbilical artery and descending aorta. Reversed end-diastolic flow velocity is apparent in the external iliac artery, whereas continuous diastolic flow characterizes the uterine and arcuate vessels. Finally, note the greatly diminished end-diastolic flow in the fetal descending aorta. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 9-16 Doppler systolic–diastolic waveform indices of blood flow velocity. The mean is calculated from computer-digitized waveforms. D, diastole; S, systole. (From Low JA: The current status of maternal and fetal blood flow velocimetry. Am J Obstet Gynecol 164(4):1049–1063, 1991. Copyright Elsevier 1991.)
Umbilical Artery Doppler
The umbilical artery normally has forward flow throughout the cardiac cycle, and the amount of flow during diastole increases as gestation advances. The S/D ratio decreases from about 4.0 at 20 weeks to about 2.0 at 40 weeks. It is considered abnormal if elevated above the 95th percentile for gestational age or if diastolic flow is either absent or reversed (see Figure 9-17). Reversed flow is an ominous finding that indicates extreme downstream resistance, placental dysfunction, and fetal circulatory compromise. Umbilical artery Doppler has been subjected to more rigorous assessment than has any previous test of fetal health and is considered to be a useful adjunct in the management of pregnancies complicated by fetal growth restriction. It is not recommended for screening of low-risk pregnancies.
FIGURE 9-17 Umbilical artery Doppler waveforms. A. Normal diastolic flow. B. Absence of end-diastolic flow. C. Reversed end-diastolic flow. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Ductus Arteriosus Doppler
Doppler evaluation of the ductus arteriosus has been used primarily to monitor fetuses exposed to indomethacin or other nonsteroidal inflammatory agents (NSAIDS), as these may cause fetal ductal constriction or closure. The resulting increased pulmonary flow may cause reactive hypertrophy of the pulmonary arterioles and development of pulmonary hypertension. The ductal construction can be reversed and is related to dosage and duration of administration. Thus, NSAID administration is typically limited to less than 72 hours, and those on NSAIDS are closely monitored for ductal constriction.
Middle Cerebral Artery (MCA) Doppler
Doppler measurement of the middle cerebral artery has been primarily employed for detection of fetal anemia. With fetal anemia, the peak systolic velocity is increased due to increased cardiac output and decreased blood viscosity. Anatomically, the path of the MCA is such that flow approaches the transducer “head on,” allowing the angle of insonation to be kept low for accurate measurement of velocity (see Figure 9-18). This has permitted the reliable, noninvasive detection of fetal anemia in cases of blood-group alloimmunization. In most centers, MCA peak systolic velocity has replaced invasive testing with amniocentesis for fetal anemia detection.
FIGURE 9-18 Middle cerebral artery color Doppler (A) and waveform (B) in a 32-week fetus with elevated peak systolic velocity secondary to fetal anemia from Rh alloimmunization. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
M-mode, or motion-mode sonography is a linear display of the events of the cardiac cycle, with time on the x-axis and motion on the y-axis. It is commonly used to measure the heart rate and allows separate evaluation of atrial and ventricular waveforms when an arrhythmia is present (see Figure 9-19). It can be used to assess ventricular function and atrial and ventricular outputs, as well as the timing of these events.
FIGURE 9-19 M-mode with superimposed color Doppler demonstrates the normal concordancet between the atrial and ventricular contractions. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 16, “Fetal Imaging.”