Maternal serum alpha-fetoprotein
A high maternal serum alpha-fetoprotein (≥2 multiples of the median for the appropriate week of gestation) between 15 and 20 weeks gestation carries a relative risk of 224 for neural tube defects, and an abnormal value (high or low) was associated with 34% of all major congenital defects 1).
The sensitivity of maternal serum AFP for spina bifida was 91% (10 of 11 cases), it was 100% for 9 cases of anencephaly. However, other series show a lower sensitivity. Closed lumbosacral spine defects, accounting for ≈ 20% of spina bifida patients, 2) will probably be missed by serum AFP screening, and may also be missed on ultrasound. Since maternal serum AFP rises during normal pregnancy, an overestimate of gestational age may cause an elevated AFP to be interpreted as normal, and an underestimate may cause a normal level to be interpreted as elevated 3).
A 2nd-trimester maternal serum alpha-fetoprotein (MSAFP) level >3.8MoM in a fetus with an open neural tube defect (ONTD) is associated with mid-gestation ventriculomegaly 4).