Elevated Maternal Serum AFP (MSAFP) at 16 Weeks
Jun 2010 – by M. Mennuti, MD
Alpha-fetoprotein is produced by the fetal liver. Levels of maternal serum alpha-fetoprotein (MSAFP) rise during the second trimester of pregnancy. Values are expressed as MoMs (multiples of the median) for each specific week of gestation. Values greater than 2.5 MoMs are generally considered elevated. Correction factors for maternal weight, race and diabetes are often used in the calculation of the MoMs. MSAFP is often elevated when there is an open fetal defect (e.g spina bifida, anencephaly, abdominal wall defect, etc). In contrast, the median MSAFP in women carrying a fetus with Down syndrome is reduced by about 25% when compared to normal controls.
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 screening for Down syndrome. Errors in dating which underestimate the gestational age may result in false positive elevations (i.e. the patient is actually 20 weeks gestation, but thought to be 16 weeks). Multiple fetuses (e.g. twins, triplets) also result in elevations of MSAFP. As the level of alpha-fetoprotein is much higher in fetal blood than in maternal blood, fetal-to-maternal bleeding can markedly elevate MSAFP.
Evaluation of a patient with an elevated MSAFP is to perform ultrasound. If errors in dating are found the MSAFP result should be reinterpreted or the test redrawn. Careful anatomic evaluation for MSAFP open defects of the spine or abdominal wall should be performed. 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 ultrasound 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 with an elevated MSAFP. A statistically significant 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.