Maternal Biochemical Screening

Low-risk women can be offered screening for genetic abnormalities of the fetus by biochemical testing in the first or second trimester and ultrasound nuchal translucency screening (first trimester) and targeted ultrasound evaluation of fetal anatomy, best done at 18 to 20 weeks' gestation. The general consensus is that women of any age should have access to any screening or diagnostic testing (as previously described) if they choose to accept the underlying risks.

Biochemical testing is the measurement of certain chemicals in the mother's blood found to be predictive of fetal abnormality. The quadruple screen is a maternal blood test in which one maternal sample drawn at 15 to 20 weeks (but most sensitive at 16 to 18 weeks) is assayed for alpha fetoprotein, estriols, and beta subunit of human chorionic gonadotropin (hCG). Normal ranges vary for each gestational week. Maternal serum alpha-fetoprotein (MSAFP) elevations can result from open neural tube defects such as spina bifida or anencephaly, when the protein leaks from the fetal tissue into the amniotic fluid through the amniocho-rion and into the maternal system. A lower-than-expected MSAFP suggests Down syndrome. The addition of p subunit of hCG and estriols has improved sensitivity for the detection of chromosomal abnormalities (Table 21-5). This test should be offered to any pregnant women with appropriate counseling regarding sensitivity and specificity. Women with abnormal tests should be referred for targeted ultrasound and possibly amniocentesis.

First-trimester ultrasound and biochemical screening are now available for clinical use, specifically the "First Trimester Screen" performed between 10 weeks, 2 days and 13 weeks, 6 days of gestation. Testing involves an ultrasound measurement of the nuchal translucency (lymphatic fluid at fetal neck) (Fig. 21-4) and laboratory measurement of pregnancy-associated plasma protein A (PAPP-A) and p subunit

Figure 21-4 Transabdominal ultrasound assessment of nuchal translucency (NT). The measurement in this figure is normal. An increased measure (>3 mm) is associated with an increased risk of chromosomal abnormalities, complex congenital heart disease, and other genetic syndromes.

Table 21-5 Second-Trimester Quadruple-Screen Results

Biochemical Results

Table 21-5 Second-Trimester Quadruple-Screen Results

Biochemical Results

Fetoprotein

Estriols

Inhibin ß-HCG A

Down syndrome

Decreased

Decreased

Increased Increased

Trisomy 18

Decreased

Decreased

Decreased No change to decreased

Open neural tube defect"

Increased

Capabilities Statistics

Detection Sensitivity

Positive False- Predictive Positive Rate Value

Down syndrome

77%-79%

3%-5%

3.7%

Trisomy 18

60%

2%-4%

2.2%

Open neural tube defect"

90%

4.0%

2.5%

*Most sensitive at 16 to 18 weeks of gestation. **Not used for neural tube defect screening. HCG, Human chorionic gonadotropin.

of hCG. The First Trimester Screen has about 85% sensitivity for Down syndrome detection at 4% false-positive rate. Several tests are also available that combine a first-trimester screen with a second-trimester screen for even higher sensitivity. Physicians should become familiar with the sensitivity, specificity, and availability of the test most suitable for their practices.

EVIDENCE-BASED SUMMARY

• First-trimester screening using both nuchal translucency measurement and biochemical markers is an effective screening test for Down syndrome in the general population (SOR: A).

• Women found to have increased risk of aneuploidy with firsttrimester screening should be offered genetic counseling and the option of chorionic villus sampling or second-trimester amniocen-tesis (SOR: A).

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