Maternal Serum AFP Elevation: Neural-Tube Defect Screening. All pregnant women are ofered screening for fetal open neural-rube defects in the second trimester, either with MSAFP screening or with sonography (American College of Obstetricians and Gynecologists, 20 16c). Measurement of the MSAFP concentration between 15 and 20 weeks' gestation has been ofered as part of routine prenatal care for more than 30 years. Because AFP is the major protein in fetal serum, analogous to albumin in a child or adult, the normal concentration gradient between fetal plasma and maternal serum is on the order of 50,000: 1. Defects in fetal integument, such as neural-tube and ventral-wall defects, permit AFP to leak into the amnionic fluid, resulting in dramatically increased maternal serum levels. he AFP value rises by about 15 percent per week during the screening window (Knight, 1992). he MoM is generally recalculated if the irst-trimester CRL or second-trimester biparietal diameter difers from the stated gestational age by more than 1 week.
Using an MSAFP level of 2.5 MoM as the upper limit of normal, the neural-tube defect detection rate is at least 90 percent for anencephaly and 80 percent for spina biida, with a screen-positive rate of 3 to 5 percent (American College of Obstetricians and Gynecologists, 2016a; Milunsky, 2004). Higher screening threshold values are used in twin pregnancies (Cuckle, 1990).
Virtually all cases of anencephaly and many cases of spina biida may be detected or suspected during a standard second-trimester sonographic examination (Dashe, 2006). Most centers now use targeted sonography as the primary method to evaluate elevated MSAFP levels and as the prenatal diagnostic test of choice for neural-tube defects (Chap. 10, p. 192). If targeted sonography is not available and myelomeningocele cannot be excluded, amniocentesis may be considered for measurement of amnionic luid AFP and acetylcholinesterase levels. That said, we recommend additional imaging prior to establishing the diagnosis, with the understanding that other
TABLE 14-6. Conditions Associated with an Elevated MSAFP Concentration
Renal anomalies-polycystic kidneys, renal agenesis, congenital nephrosis, urinary tract obstruction
Chorioangioma of placenta
MSAFP = maternal serum alpha-fetoprotein.
abnormalities or conditions can result in elevation of these amnionic luid analytes (Table 14-6). Sonographic indings characteristic of fetal neural-tube defects are reviewed in Chapter 10 (p. 192). Fetal surgery for myelomeningocele is discussed in Chapter 16 (p. 319).
Unexplained Abnormalities of Second-Trimester Analytes.