The incidence of major abnormalities apparent at birth is 2 to 3 percent. These anomalies cause a significant portion of neonatal deaths, and more than a fourth of all pediatric hospital admissions result from genetic disorders (Lee and colleagues, 2001). Prenatal diagnosis is the science of identifying structural or functional abnormalities—birth defects—in the fetus. With this information, clinicians hope to provide appropriate counseling and optimize outcome. In some cases, fetal therapy can be used to improve the intrauterine environment. Therapy may include blood product transfusion, administration of medication transplacentally or via the fetal circulation, laser or radiofrequency ablation of vascular anastomoses, amnioreduction, shunt placement, or more extensive fetal surgery.
Birth defects can arise in at least three ways. The most common type of structural fetal abnormality is a malformation—an intrinsic abnormality “programmed” in development, regardless of whether a precise genetic etiology is known. An example is spina bifida. The second type is a deformation caused when a genetically normal fetus develops abnormally because of mechanical forces imposed by the uterine environment. An example is an otherwise normal limb that develops contractures because of prolonged oligohydramnios. The third type is a disruption, which is a more severe change in form or function that occurs when genetically normal tissue is modified as the result of a specific insult. An example is damage from an amnionic band causing a cephalocele or limb-reduction abnormality.
Sometimes multiple structural or developmental abnormalities occur together in one individual. A cluster of several anomalies or defects can be a syndrome, meaning that all the abnormalities have the same cause—for example, trisomy 18 (see Chap. 12, Trisomy 18). Anomalies also may comprise a sequence, meaning that all developed sequentially as result of one initial insult—for example, oligohydramnios leading to pulmonary hypoplasia, limb contractures, and facial deformities. Finally, a group of anomalies may be considered an association, in which particular anomalies occur together frequently but do not seem to be linked etiologically—for example, VATER, association of vertebral defects, anal atresia, tracheoesophageal fistula with esophageal atresia, and radial dysplasia. It is readily apparent that classification of fetal anomalies can be challenging, and reclassification may be required.
Neural-Tube Defects (NTDs)
The open neural-tube defects include anencephaly, spina bifida, cephalocele, and other rare spinal fusion (schisis) abnormalities. Features of these anomalies are reviewed in Chapter 16 (Normal and Abnormal Fetal Anatomy). As a class, these defects of neurulation occur in 1.4 to 2 per 1000 pregnancies and are the second most common class of birth defect after cardiac anomalies (American College of Obstetricians and Gynecologists, 2003). In the 1970s, Brock and associates (1972, 1973) observed that pregnancies complicated by a NTD had higher levels of alpha-fetoprotein (AFP) in maternal serum and amnionic fluid. This formed the basis for the first maternal serum screening test for a birth defect.