Elevated Maternal Serum AFP (MSAFP) at 16 Weeks
Editor: Roger P. Smith, MD
Alpha-fetoprotein is produced by the fetal liver. Levels rise during the second trimester. Values are expressed as MoMs (multiples of the median) for each specific week of gestation, and values greater than 2.5 MoMs are generally considered elevated. MOMS are typically corrected for maternal weight, race, and diabetes. MSAFP is often elevated when there is an open fetal defect such as spina bifida, anencephaly, or an abdominal wall defect. The median MSAFP is reduced about 25% in women carrying a fetus with Trisomy 21 when compared to pregnancies with a normal chromosomal complement.
Measurement of MSAFP at 16 weeks was initially used to screen for open neural tube defects. It is also commonly used as one of the analytes in second trimester serum screening for Trisomy 21. Errors in dating which underestimate the gestational age (e.g. the patient is actually 20 weeks gestation, but thought to be 16 weeks) may result in false positive elevations. Multiple gestations also cause elevations of MSAFP. The level of alpha-fetoprotein is much higher in fetal than maternal blood, so fetal-to-maternal bleeding can markedly elevate MSAFP.
Patients with an elevated MSAFP should be initially evaluated with ultrasonography . If errors in dating are found, the MSAFP result should be reinterpreted or the test redrawn. Elevated levels of MSAFP should prompt careful anatomic evaluation for open defects of the spine or abdominal wall. Intracranial anatomic changes (e.g. “lemon sign”, “banana sign”) associated with spina bifida should also be evaluated. Amniocentesis to measure amniotic fluid AFP may be considered. Many experts believe that improvement in ultrasonography has largely supplanted the value of performing amniocentesis primarily to measure amniotic fluid AFP. Testing for fetal-to-maternal bleeding by Kleihauer-Betke staining may be considered, particularly in Rh negative patients.
An association between elevated MSAFP and low birth weight or poor obstetric outcome has been reported. The value of this association in the management of the patient is limited because of the low sensitivity and low positive predictive value of this finding. There are currently no recommendations to modify pregnancy or delivery management based on finding an unexplained elevated MS-AFP in an otherwise normal pregnancy.
American College of Obstetricians and Gynecologists. Practice Bulletin No. 163 Summary: Screening for Fetal Aneuploidy. Obstet Gynecol. 2016 May;127(5):979-81. doi: 10.1097/AOG.0000000000001439.
Original approval June 2010; revised September 2016; reaffirmed March 2018
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