Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 3. Prenatal Diagnosis

Prenatal diagnosis is the science of identifying structural or functional abnormalities in the fetus. The incidence of major abnormalities apparent at birth is 2 to 3 percent. The vast majority of cases of neural-tube defects (NTDs), Down syndrome, and many other fetal abnormalities occur in families with no prior history of birth defects. Couples with no family history of genetic abnormalities are routinely offered screening tests for certain fetal disorders. Screening tests do not provide a diagnosis, but rather identify individuals whose risk is high enough to benefit from a definitive diagnostic test. Procedures used in prenatal diagnosis are reviewed in Chapter 4.

NEURAL-TUBE DEFECTS

As a class, neural-tube defects occur in 1.4 to 2 per 1000 pregnancies and are the second most common class of birth defect after cardiac anomalies. Features of NTDs are reviewed in Chapter 9 (pp. 70–71). Almost 95 percent of NTDs occur in pregnancies without a recognized risk factor or family history—hence the need for routine screening. There are, however, specific risk factors, some of which are listed in Table 3-1. Many women at increased risk for NTD benefit from taking 4 mg of folic acid daily before conception and through the first trimester. These include individuals with one or more prior affected children or if either the pregnant woman or her partner has an NTD. In low-risk women, serum screening for NTDs is done with maternal serum alpha-fetoprotein (AFP). Diagnostic tests include specialized sonography (Chapter 9, pp. 68) and amniocentesis.

TABLE 3-1. Some Risk Factors for Neural-Tube Defects

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Maternal Serum AFP Screening

The American College of Obstetricians and Gynecologists recommends that all pregnant women be offered second-trimester maternal serum AFP screening. This is a component of multiple marker serum screening and is generally offered between 15 and 20 weeks. Maternal serum AFP is reported as a multiple of the median (MoM) of the unaffected population, which normalizes the distribution of AFP levels and permits comparison of results from different laboratories and populations. Using a level of 2.0 or 2.5 MoM as the upper limit of normal, most laboratories report a detection rate (test sensitivity) of at least 90 percent for anencephaly and 80 percent for spina bifida, at a screen positive rate of 3 to 5 percent. The reason that the detection rate is not higher is explained by the overlap in AFP distributions in affected and unaffected pregnancies, as shown in Figure 3-1. The positive predictive value, those with AFP elevation who have an affected fetus, is only 2 to 6 percent.

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FIGURE 3-1 Maternal serum alpha-fetoprotein distribution for singleton pregnancies at 15 to 20 weeks. The screen cut-off value of 2.5 multiples of the median is expected to result in a false-positive rate of up to 5 percent (black-hatched area) and false-negative rates of up to 20 percent of spina bifida (tan-hatched area) and 10 percent for anencephaly (red hatched area). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Factors that influence the AFP result include maternal weight, gestational age, race, diabetes, and multifetal gestation. One algorithm for evaluating maternal serum AFP is shown in Figure 3-2. Because underestimation of gestational age, multiple gestation, and fetal death may cause AFP to be abnormally elevated, evaluation begins with a standard ultrasound examination if not already performed. In addition to NTDs, many other types of birth defects and placental abnormalities are associated with AFP elevation (Table 3-2). The likelihood that a pregnancy is affected by a fetal or placental abnormality increases in proportion to the AFP level. Women confirmed to have serum AFP elevation should be referred for counseling and consideration of a diagnostic test.

TABLE 3-2. Some Conditions Associated with Abnormal Maternal Serum Alpha-Fetoprotein Concentrations

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FIGURE 3-2 Example of an algorithm for evaluating maternal serum alpha-fetoprotein screening values (MSAFP). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al, eds. Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Specialized Sonography

Many centers use specialized or targeted sonography as the primary method of evaluating an elevated serum AFP level. Anencephaly, other cranial defects, and most spine defects can be readily identified (Figures 9-2 and 9-3). Open spina bifida is associated with inward scalloping of the frontal bones, also called the lemon sign (Figure 3-3), and downward herniation of the cerebellum with effacement of the cisterna magna, also called the banana sign(Figure 3-4). Overall NTD risk may be reduced by at least 95 percent when no spine or cranial abnormalities are observed, with experienced investigators describing nearly 100 percent detection of open NTDs.

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FIGURE 3-3 In this axial image of the fetal head at the level of the lateral ventricles, inward bowing or scalloping of the frontal bones (arrows) in the setting of spina bifida produces the “lemon sign.” The image also depicts ventriculomegaly. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

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FIGURE 3-4 In this image of the fetal head at the level of the posterior fossa, downward herniation of the cerebellum (white arrows), with effacement of the cisterna magna, produces the “banana sign.” (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Amniocentesis

Until relatively recently, an elevated serum AFP level prompted amniocentesis to determine the amnionic fluid AFP level, and, if elevated, an assay for amnionic fluid acetylcholinesterase. If both are elevated, the overall sensitivity is about 98 percent for open NTDs, with a false positive rate of 0.4 percent. Although amniocentesis is offered, many women instead opt for specialized sonography. The American College of Obstetricians and Gynecologists recommends that women be counseled regarding the risks and benefits of both diagnostic tests, the risk associated with their degree of AFP elevation or other risk factors, and the quality and findings of the sonographic examination before making a decision. If amniocentesis is elected, fetal karyotype may be considered.

Unexplained Maternal Serum AFP Elevation

When no fetal or placental abnormality is detected after a specialized sonographic evaluation, with or without amniocentesis, the AFP elevation is considered unexplained. These pregnancies are at increased risk for a variety of subsequent adverse outcomes, including a fetal anomaly that may not be detectable prenatally (Table 3-2), fetal growth restriction, oligohydramnios, placental abruption, preterm birth, and even fetal death. No specific program of maternal or fetal surveillance has been found to favorably affect the pregnancy outcome in such cases, and fortunately, most women with unexplained AFP elevation have a normal outcome.

SCREENING FOR FETAL ANEUPLOIDY

The risk of fetal trisomy increases considerably with maternal age and rises most rapidly beginning at age 35 (Table 3-3 and Figure 5-1). Traditionally, age 35 was selected as the cut-off for “advanced maternal age,” the age at which prenatal diagnostic tests for aneuploidy such as amniocentesis would be offered (Chapter 4). With the development of screening tests for Down syndrome, younger women were offered amniocentesis if their risk was the same or greater than that of a woman 35 years of age at delivery. During the past two decades, the field of prenatal diagnosis has undergone major advances, with increased sensitivity of second-trimester aneuploidy screening and the development of accurate first-trimester screening (Table 3-4). Because of this, the American College of Obstetricians and Gynecologists recommends that all women who present for prenatal care before 20 weeks be offered aneuploidy screening. Regardless of age, all women are counseled regarding the differences between screening and diagnostic tests, and they are given the option of invasive diagnostic testing. A positive screening test indicates increased risk, but it is not diagnostic of Down syndrome or other aneuploidy. Conversely, a negative screening test indicates that the risk is not increased, but it does not guarantee a normal fetus.

TABLE 3-3. Singleton Gestation—Maternal Age-Related Risk for Down Syndrome and Any Aneuploidy at Midtrimester and Term

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TABLE 3-4. Selected Down Syndrome Screening Strategies

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Second-Trimester Screening

At 15 to 20 weeks, Down syndrome pregnancies are characterized by a low maternal serum AFP level, elevated human chorionic gonadotropin (hCG), and a low level of unconjugated estriol. This triple testcan detect 65 to 70 percent of Down syndrome cases and can also screen for trisomy 18, in which all three serum markers are decreased. A fourth marker, dimeric inhibin, has been added to make the quadruple or quad test. Dimeric inhibin is elevated in Down syndrome pregnancies. The quad test can detect 80 percent of Down syndrome cases. Accurate gestational age is essential to achieve accurate aneuploidy detection with these screening tests. Once gestational age is confirmed by ultrasound, women with a positive screening test are offered amniocentesis or fetal blood sampling for fetal karyotype (discussed in Chapter 4).

First-Trimester Screening and Combined Screening

First-trimester aneuploidy screening is performed between 11 and 14 weeks. The most commonly used protocol combines the fetal nuchal translucency (NT), discussed in Chapter 9, with two serum analytes, hCG and pregnancy-associated plasma protein A (PAPP-A). NT measurement should be performed only by operators with specific training and ongoing monitoring, and only when counseling and early invasive fetal testing are available. Down syndrome pregnancies are characterized by increased fetal NT, elevated hCG, and lower PAPP-A. Using these three markers, first-trimester Down syndrome detection is comparable to that with second-trimester quad screening, and detection of trisomies 18 and 13 is as high as 90 percent. If the NT measurement is abnormally increased, approximately a third of fetuses will have a chromosome abnormality, and half of these are Down syndrome. Despite this, detection of Down syndrome is significantly greater when NT is used in conjunction with serum markers, and for this reason, use of NT alone is recommended only in selected circumstances—for example, multifetal gestation. There is also a strong association between increased NT and fetal cardiac anomalies. When the nuchal translucency measurement is 3.5 mm or greater with a normal fetal karyotype, then targeted sonography, fetal echocardiography, or both should be considered.

As first-trimester screening has become incorporated into clinical practice, research efforts have focused on further improving screening efficacy by combining the currently available first- and second-trimester screening technologies. A number of combined screening strategies have been developed (see Table 3-4). Integrated screening, which combines results of both first- and second-trimester screening tests into a single risk, has the highest Down syndrome detection—90 to 96 percent—but has the disadvantage that results are not available until the second-trimester screening test has been completed. Sequential screening discloses results of first-trimester screening to women at highest risk. There are two types. With stepwise sequential screening, women at highest risk, for example, the top 1 percent, are informed of their results and offered invasive testing, while the remainder undergo second-trimester screening. With contingent sequential screening, women are divided into three groups: high, moderate, and low risk. Those at highest risk are counseled and offered invasive testing, those at lowest risk have no further testing, and only women at moderate risk—about 15 to 20 percent—go on to second-trimester screening. Integrated and sequential screening strategies require coordination between the practitioner and the laboratory to ensure that the second sample is obtained during the appropriate gestational window, sent to the same laboratory, and appropriately linked to first-trimester results.

Sonographic Screening for Aneuploidy

Major fetal anomalies are often discovered in otherwise low-risk pregnancies during ultrasonography performed for other indications. An isolated malformation may be a part of a genetic syndrome, and if so, the fetus may have other abnormalities undetectable by ultrasonography that affect the prognosis—such as mental retardation. With few exceptions, the specific aneuploidy risk associated with most major anomalies is high enough to merit offering invasive fetal testing (Table 3-5). Although the finding of a major anomaly often increases the aneuploidy risk, it should not be assumed that aneuploid fetuses will have a sonographically detectable major malformation. For example, only 25 to 30 percent of second-trimester fetuses with Down syndrome will have a major malformation that can be identified sonographically.

TABLE 3-5. Aneuploidy Risk Associated with Selected Major Fetal Anomalies

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Sonographic detection of aneuploidy, particularly Down syndrome, may be increased by the minor sonographic markers, which have been collectively referred to as soft signs. In the absence of aneuploidy or an associated major malformation, these markers usually do not affect the prognosis. Those shown in Table 3-6 have been the focus of genetic sonogram studies, in which likelihood ratios for Down syndrome have been calculated. The aneuploidy risk increases with the number of markers identified. The incorporation of minor markers into second-trimester screening protocols has been studied largely in high-risk populations, with reported detection of Down syndrome of 50 to 75 percent. Unfortunately, between 10 and 15 percent of unaffected pregnancies will have one of these markers, significantly limiting their utility for general population screening.

TABLE 3-6. Likelihood Ratios and False-Positive Rates for Isolated Second-Trimester Markers Used in Down Syndrome Screening Protocols

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FAMILIAL GENETIC DISEASE

Couples with a personal or family history of a heritable genetic disorder should be offered genetic counseling and provided with a calculated or estimated risk of having an affected fetus. Some otherwise rare recessive genes are found with increased frequency in certain racial or ethnic groups (Table 3-7). A phenomenon called the founder effect occurs when an otherwise rare gene that is found with increased frequency within a certain population can be traced back to a single family member or small group of ancestors.

TABLE 3-7. Autosomal Recessive Diseases Found with Increased Frequency in Certain Ethnic Groups

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Cystic Fibrosis (CF)

CF is an autosomal recessive disease caused by a mutation in a gene on chromosome 7 that encodes the cystic fibrosis conductance transmembrane regulator (CFTR) protein. More than 1500 mutations have been described. Although phenotype prediction is fairly accurate if the mutations are ΔF508 or W1282X, other mutations are less closely associated with disease symptoms, and phenotype prediction is difficult. The American College of Obstetricians and Gynecologists recommends that information about CF screening be made available to all couples, and the current screening panel contains 23 pan-ethnic CF gene mutations. Both the risk of carrying a CF gene mutation and the detection rate for the test vary by racial and ethnic group, as shown in Table 3-8. Although a negative test does not preclude the possibility of carrying another mutation, it does reduce the risk substantively from the background rate. When both partners are from higher risk groups, carrier screening should be offered before conception or early in pregnancy. For individuals with a family history of CF, it is helpful to obtain records of the CFTR mutation. If the mutation has not been identified, screening with an expanded panel or even complete CFTR gene sequencing may be necessary.

TABLE 3-8. Cystic Fibrosis Carrier Risk by Racial and Ethnic Group, Before and After Testing

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Diseases in Individuals of Ashkenazi Jewish Descent

The American College of Obstetricians and Gynecologists recommends that Ashkenazi Jewish individuals be offered carrier screening for cystic fibrosis, Tay-Sachs disease, Canavan disease, and familial dysautonomia, either before conception or during early pregnancy. This is because of their relatively high prevalence, consistently severe and predictable phenotype, and high detection rate in this population. For other conditions that are also more common in Ashkenazi Jewish individuals (Table 3-7), patient education materials can be made available so that those who are interested can request additional information or carrier screening.


For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 13, “Prenatal Diagnosis and Fetal Therapy.”