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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Prenatal Genetic Screening – Serum and Ultrasound
Sponsored by The Indian Health Service Clinical Support Center
4. Neural Tube Defects: Maternal serum alpha fetoprotein
Case Scenario
RWA is a 40 y/o G6P5 at 16 weeks gestation. She is counseled about her advanced maternal age (AMA) risks and decides she wishes to have multiple marker screening performed. Is this an appropriate strategy for her?
Neural Tube Defects: Maternal serum alpha fetoprotein
Maternal serum alpha fetoprotein (AFP) screening alone will detect about 75% of neural tube defects. Of the women whose tests return positive for neural tube defects, about 1 in 50 will have a fetus with a NTD.
A screen that is positive for NTD may suggest many other conditions or be a false positive. Among the other fetal conditions that may be associated with an elevated AFP are the ventral wall defects (gastroschisis and omphalocele). These conditions are potentially remediable after birth, and they have other perinatal implications. Other more common associations with elevated AFP are wrong dates, twins, early pregnancy bleeding, and fetal demise. By looking for these other conditions, ultrasound obviously plays an important role in any patient with an abnormal screen.
AFP peaks at about 12 weeks of gestation. Data on normal values for the MMS have been accumulated for the gestational age window of 15 to 20 weeks. If the gestational age is miscalculated and the test it is drawn too early a spuriously elevated AFP value may be obtained. In order to evaluate positive results an ultrasound should be ordered expeditiously. If the gestational age derived from the ultrasound differs by more than 10 days from the dates as calculated by the patient’s LMP, the reference laboratory must be requested to recalculate the result, a process that may take one to several days.
Borderline elevated results (2.5 to 3.0 multiples of the median, or “MoM”, depending on your lab) should be repeated once an accurate gestational age is established. If the repeat value is normal, which occurs about a third of the time, nothing further need be done. This is not the strategy that should be employed for a screen that is positive for Down syndrome, however.
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