Careful examination should ordinarily lead to a correct diagnosis of hydrocephalus in the last weeks of pregnancy. In many cases the deformity can be detected by external palpation.
—J. Whitridge Williams (1903)
In the initial edition of Williams Obstetrics, very few fetal disorders could be identified before delivery. Now, more than 100 years later, prenatal diagnosis has become a separate field of its own. Strictly speaking, prenatal diagnosis is the science of identifying congenital abnormalities, aneuploidies, and other genetic syndromes in the fetus. It encompasses the diagnosis of structural malformations with specialized sonography; routine screening tests for aneuploidy and neural-tube defects; diagnostic tests such as karyotyping and chromosomal microarray analysis performed on chorionic villi and amniocentesis specimens; and additional screening and diagnostic tests offered to those with pregnancies at risk for specific genetic disorders. The goal of prenatal diagnosis is to provide accurate information regarding short- and long-term prognosis, recurrence risk, and potential therapy, thereby improving patient counseling and optimizing outcomes.
Management of an affected pregnancy, including whether a woman would elect pregnancy termination, may be incorporated into the discussion of screening and testing options. However, nondirective counseling is central to prenatal diagnosis. This practice provides the patient with unbiased knowledge regarding a diagnosis and preserves her autonomy (Flessel, 2011). Fetal imaging of congenital anomalies is discussed in Chapter 10, and pregnancy termination is discussed in Chapter 18.
More than 40 years ago, Brock (1972, 1973) observed that pregnancies complicated by neural-tube defects 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 fetal condition. The beginning of widespread serum screening came in 1977, after a collaborative trial from the United Kingdom established the association between elevated maternal serum AFP levels (MSAFP) and fetal open neural-tube defects (Wald, 1977). When screening was performed at 16 to 18 weeks’ gestation, detection approached 90 percent for pregnancies with fetal anencephaly and 80 percent for those with myelomeningocele (spina bifida). These sensitivities are comparable to current testing (American College of Obstetricians and Gynecologists, 2016a).
The terms level I and level II sonography were coined in this context. In the California MSAFP Screening Program of the 1980s and early 1990s, women received serum screening prior to sonography, and those with an elevated MSAFP level would undergo level I sonography to identify an incorrect gestational age, multifetal gestation, or fetal demise (Filly, 1993). A third of pregnancies with an elevated MSAFP level had one of these three etiologies. Although birth defects were occasionally detected during level I sonography, this was not the expectation. If level I sonography did not identify an etiology for the MSAFP level elevation, amniocentesis would be offered. Then, only if the amnionic fluid AFP concentration were elevated would the woman ...