The purpose of prenatal screening and diagnosis is not just to detect fetal abnormalities but also to allay anxiety and provide assistance to prepare for a child with a specific disability. Prenatal screening cannot be used to rule out all possible fetal abnormalities. It is limited to determining whether the fetus has (or probably has) designated conditions indicated by late maternal age, family history, or well-defined risk factors.
There are multiple methods that can assist in diagnosing a fetus regarding genetic disorders, including ultrasonography, maternal serum (blood) screening tests, amniocentesis, chorionic villus sampling, and percutaneous umbilical fetal blood sampling (Fig. 7.14). Prenatal diagnosis can also pro- vide the information needed for prescribing prenatal treatment for the fetus. For example, if congenital adrenal hyperplasia is diagnosed, the mother can be treated with adrenal cortical hormones to prevent masculinization of a female fetus.
Ultrasonography is a noninvasive diagnostic method that uses reflections of high-frequency sound waves to visualize soft tissue structures. Since its introduction in 1958, it has been used during pregnancy to determine the number of fetuses, fetal size and position, amount of amniotic fluid, and placental location. It also is possible to assess fetal movement, breathing movements, and heart pattern. There is also good evidence that early ultrasonography (i.e., before 14 weeks) accurately determines gestational age.
Improved resolution and real-time units have enhanced the ability of ultrasound scanners to detect congenital anomalies. Ultrasonography makes possible the in utero diagnosis of cardiac defects, hydrocephalus, spina bifida, facial defects, congenital heart defects, congenital diaphragmatic hernias, disorders of the gastrointestinal tract, skeletal anomalies, and various other defects. Three-dimensional (3D) sonography has become useful in better assessing facial profiles and abdominal wall defects. A fetal echocardiogram can be done as follow-up for possible cardiac anomalies. Fetal MRI can be done to better assess skeletal, neurological, and other anomalies. Intrauterine diagnosis of congenital abnormalities permits better monitoring, further workup and planning with appropriate specialties, preterm delivery for early correction, selection of cesarean section to reduce fetal injury, and, in some cases, intrauterine therapy.
Maternal Serum Markers
Maternal blood testing began in the early 1980s with the test for AFP. Since that time, a number of serum factors have been studied as screening tests for fetal anomalies.
Current maternal testing favors first trimester screening for all women between 11 and 13 weeks combining nuchal translucency seen on sonogram with PAPP-A level, hCG level, and maternal age to determine a risk for trisomy 21, 13, and 18. PAPP-A, which is secreted by the placenta, has been shown to play an important role in promoting cell differentiation and proliferation in various body systems. In complicated pregnancies, the PAPP-A concentration increases with gestational age until term. Decreased PAPP-A levels in the first trimester (between 10 and 13 weeks) have been shown to be associated with Down syndrome. When used along with maternal age, free β-hCG, and ultrasonographic measurement of nuchal translucency, serum PAPP-A levels can report-edly detect 85% to 95% of affected pregnancies with a false-positive rate of approximately 5%.
A maternal serum AFP can then be done alone in the second trimester to assess for NTDs, though for pregnant women with access to good quality sonography centers, a level II ultrasound for anatomical viewing of the spine can exclude greater than 99% of spinal defects.
For pregnant women presenting too late for first trimester screening, the quad screen using AFP, hCG, inhibin A, and unconjugated estriol is used to screen for trisomy and NTDs between 15 and 22 weeks of pregnancy. The use of ultrasonography to verify fetal age can reduce the number of false- positive tests with this screening method.
AFP is a major fetal plasma protein and has a structure similar to the albumin found in postnatal life. AFP is made initially by the yolk sac, gastrointestinal tract, and liver. Fetal plasma levels of AFP peak at approximately 10 to 13 weeks’ gestation and decrease until the third trimester when the level peaks again. Maternal and amniotic fluid levels of AFP are elevated in pregnancies where the fetus has an NTD (i.e., anencephaly and open spina bifida) or certain other mal-formations such as an anterior abdominal wall defect in which the fetal integument is not intact. Although NTDs have been associated with elevated levels of AFP, decreased levels have been associated with Down syndrome.
A complex glycoprotein, hCG, is produced exclusively by the outer layer of the trophoblast shortly after implantation in the uterine wall. It increases rapidly in the first 8 weeks of gestation, declines steadily until 20 weeks, and then plateaus. The single maternal serum marker that yields the highest detection rate for Down syndrome is an elevated level of hCG. Inhibin A, which is secreted by the corpus luteum and fetoplacental unit, is also a maternal serum marker for fetal Down syndrome.
Unconjugated estriol is produced by the placenta from precursors provided by the fetal adrenal glands and liver. It increases steadily throughout pregnancy to a higher level than that normally produced by the liver. Unconjugated estriol levels are decreased in Down syndrome and trisomy 18.
Amniocentesis is an invasive diagnostic procedure that involves the withdrawal of a sample of amniotic fluid from the pregnant uterus usually using a transabdominal approach (see Fig. 7.14). The procedure is useful in women with elevated risk on first trimester screen or quad screen, abnormal fetal findings on sonogram, or in parents who are carriers or with a strong family history of an inherited disease. Ultrasonography is used to gain additional information and to guide the placement of the amniocentesis needle. The amniotic fluid and cells that have been shed by the fetus are studied. Amniocentesis can be performed on an outpatient basis starting at 15 weeks. For chromosomal analysis, the fetal cells are grown in culture and the result is available in 10 to 14 days. Beyond prenatal diagnosis, amniocentesis can also be done throughout the pregnancy as needed for testing. In cases of suspected chorioamnionitis, an amniocentesis can be done to assess for infection of the amniotic fluid. Fetal lung maturity can be assessed by amniocentesis by looking for the lecithin/sphingomyelin (L/S) ratio and presence of phosphatidyl glycerol to help with delivery planning in some cases.
Chorionic Villus Sampling
Chorionic villus sampling is an invasive diagnostic procedure that obtains tissue that can be used for fetal chromosome studies, DNA analysis, and biochemical studies. Sampling of the chorionic villi usually is done after 10 weeks of gestation. Performing the test before 10 weeks is not recommended because of the danger of limb reduction defects in the fetus. The chorionic villi are the site of exchange of nutrients between the maternal blood and the embryo—the chorionic sac encloses the early amniotic sac and fetus, and the villi are the primitive blood vessels that develop into the placenta. The sampling procedure can be performed using either a transabdominal or transcervical approach (see Fig. 7.14). The fetal tissue does not have to be cultured, and fetal chromosome analysis can be made available in 24 hours. DNA analysis and biochemical tests can be completed within 1 to 2 weeks.
Percutaneous Umbilical Cord Blood Sampling
PUBS is an invasive diagnostic procedure that involves the transcutaneous insertion of a needle through the uterine wall and into the umbilical artery. It is performed under ultrasonographic guidance and can be done any time after 16 weeks of gestation. It is used for prenatal diagnosis of hemoglobinopathies, coagulation disorders, metabolic and cytogenetic disorders, and immunodeficiencies. Fetal infections such as rubella and toxoplasmosis can be detected through measurement of immunoglobulin M antibodies or direct blood cultures. Results from cytogenetic studies usually are available within 48 to 72 hours. Because the procedure carries a greater risk of pregnancy loss compared to amniocentesis, it usually is reserved for situations in which rapid cytogenetic analysis is needed or in which diagnostic information cannot be obtained by other methods. In the process of doing PUBS to assess fetal anemia, a blood transfusion can be administered to the fetus as needed.
Cytogenetic and Biochemical Analyses Amniocentesis and chorionic villus sampling yield cells that can be used for cytogenetic and DNA analyses. Biochemical analyses can be used to detect abnormal levels of AFP and abnormal biochemical products in the maternal blood and in specimens of amniotic fluid and fetal blood.
Cytogenetic studies are used for fetal karyotyping to determine the chromosomal makeup of the fetus. They are done to detect abnormalities of chromosome number and structure. Karyotyping also reveals the sex of the fetus. This may be useful when an inherited defect is known to affect only one sex.
Analysis of DNA is done on cells extracted from the amniotic fluid, chorionic villi, or fetal blood from percutaneous umbilical sampling to detect genetic defects such as inborn errors of metabolism. The defect may be established through direct demonstration of the molecular defect or through methods that break the DNA into fragments that can be studied to determine the presence of an abnormal gene. Direct demonstration of the molecular defect is done by growing the amniotic fluid cells in culture and measuring the enzymes that the cultured cells produce. Many of the enzymes are expressed in the chorionic villi. This permits earlier prenatal diagnosis because chorionic villi. This permits earlier prenatal diagnosis because the cells do not need to be subjected to prior culture. DNA studies are used to detect genetic defects that cause inborn errors of metabolism, such as Tay-Sachs disease, glycogen storage diseases, and familial hypercholesterolemia. Prenatal diagnoses are possible for more than 70 inborn errors of metabolism.
The newest realm of fetal diagnosis involves looking at fetal DNA in the maternal blood. Some private companies and many research institutions are exploring the efficacy of looking at fetal DNA for sex determination and other genetic testing. More research is needed before this will be offered to all women.