A 24-year-old white woman has a maternal serum α-fetoprotein (MSAFP) at 17 weeks gestation of 6.0 multiples of the median (MOM). Which of the following is the most appropriate next step in management?
-
A second MSAFP test
-
Ultrasound examination
-
Amniocentesis
-
Amniography
-
Recommendation of termination
explanation
The MSAFP may be performed between 15 and 21 weeks gestation to screen for neural tube defects. The recommended sequence for an MSAFP screening program for 1000 hypothetical patients would normally produce about 30 with an elevated level (2.5 MOM) on the first MSAFP. If the patient does not have an extremely elevated value (ie, the value is < 4.0 MOM) and is relatively early in pregnancy (< 19 weeks gestation), a second MSAFP value is usually drawn. About two-thirds of these patients will have an elevated test. Those who are normal a second time drop back into the normal population. However, if the value is extremely high (≥ 4.0 MOM) or if the gestational age is approaching the limit of options for termination of pregnancy (19+ weeks), most programs then skip a second test and go directly on to ultrasound and possibly amniocentesis. A thorough ultrasound on patients with two elevations or one very high elevation will reveal an obvious reason for the elevation in about 10 of 30 patients. These reasons may include anencephaly, twins, wrong gestational age of the fetus, or fetal demise. The approximately 20 patients with no obvious cause for their elevations should then be offered counseling and amniocentesis. Of patients without a benign explanation, about 5% have an elevated amniotic fluid α-fetoprotein (AFP) and positive acetylcholinesterase. Such patients will have a greater than 99% chance of having a baby with an open neural tube defect or other serious malformations, such as a ventral wall defect. Amniography is an outmoded procedure in which radiopaque dye is injected into the amniotic cavity for the purpose of taking x-rays. Under no circumstances whatsoever should termination of pregnancy be recommended on the basis of MSAFP testing alone. MSAFP is only a screening test used to define who is at risk and requires further testing; it is never diagnostic per se.