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Prenatal diagnosis is the science of identifying congenital abnormalities and genetic conditions in the fetus. It encompasses the diagnosis of structural malformations with specialized ultrasound, screening tests for aneuploidy, carrier screening for genetic diseases, and diagnostic tests performed on chorionic villi and amnionic fluid.

Based on population-based registry data including births, fetal deaths, and pregnancy terminations, 4 per 1000 pregnancies have a chromosomal abnormality (Wellesley, 2012). If chromosomal microarray analysis (CMA) is performed on chorionic villi or amnionic fluid, an additional 4 per 1000 are found to have a pathogenic copy number variant, such as a microdeletion or microduplication (Srebniak, 2017). It is important for all pregnant women to be offered both screening and diagnostic tests.

The goal of prenatal diagnosis is to provide accurate information about short- and long-term prognosis, recurrence risk, and potential therapy. Nondirective counseling and provision of unbiased knowledge are paramount. Management of an affected pregnancy that includes diagnostic testing, discussion of potential fetal therapy options and postnatal care, and decisions related to expectant management or pregnancy termination are all incorporated into counseling (Flessel, 2011). Fetal imaging of congenital anomalies is discussed in Chapter 15, fetal therapy in Chapter 19, and pregnancy termination in Chapter 11.

HISTORICAL PERSPECTIVE

Nearly 50 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. Widespread serum screening began 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). Screening at 16 to 18 weeks’ gestation detected 90 percent of fetuses with anencephaly and 80 percent of those with open spina bifida, similar to current screening performance (American College of Obstetricians and Gynecologists, 2019c).

The terms level I and level II ultrasound were coined in this context. In the California MSAFP Screening Program of the 1980s and early 1990s, the first step in evaluation of an abnormally elevated MSAFP was a level I ultrasound examination. This screening examination demonstrated that one third of pregnancies with MSAFP elevation had incorrect gestational age, multifetal gestation, or fetal demise as the etiology (Filly, 1993). Amniocentesis was then offered to the remaining two thirds. If the amnionic fluid AFP concentration was elevated, a level II ultrasound examination was performed to detect and characterize a fetal abnormality.

Additionally, an elevated amnionic fluid AFP level prompted a concurrent assay for amnionic fluid acetylcholinesterase. Because acetylcholinesterase leaks from the exposed neural tissue into the amnionic fluid, presence of both analytes was used to confirm the diagnosis. However, other fetal abnormalities are associated with elevated amnionic fluid AFP and positive assay results for acetylcholinesterase. These include ventral wall defects, esophageal atresia, fetal teratoma, cloacal exstrophy, and skin abnormalities such as ...

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