SONOGRAPHIC SCREENING ...e..e..e.e..e.e...e286
CARRIER SCREENING FOR GENETIC DISORDERS .e... 288
AMNIOCENTESIS AND CHORIONIC VILLUS SAMPLINGe. .e291
PREIMPLANTATION GENETIC TESTING .e.e.....e.. 295
Carul examination should ordinariy lead to a correct diagnosis of hydrocephalus in the last weeks of pregnancy. In many cases the dormiy can be detected by external papation.
-]. Whitridge Williams (1903)
In the initial edition of Williams Obstetrics, very few fetal disorders could be identiied 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 identiying 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 ofered to those with pregnancies at risk for speciic genetic disorders. he 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 afected 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. his 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 luid. his formed the basis for the irst 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 biida). hese sensitivities are comparable to current testing (American College of Obstetricians and Gynecologists, 20 16a).
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 identiy an incorrect gestational age, mulrifetal 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 identiy an etiology for the MSAFP level elevation, amniocentesis would be ofered. Then, only if the amnionic luid AFP concentration were elevated would the woman undergo level II sonography. his more detailed and comprehensive survey of fetal anatomy was performed to detect and characterize the fetal abnormality.