The importance of accurate gestational age estimation in screening for fetal neural tube defects using maternal serum alpha-fetoprotein levels.

Estimates of gestational age based on menstrual history are often in error, often by as much as 4 weeks. The effect of this problem with respect to a screening program for neural tube defects based on maternal serum alpha-fetoprotein determination is examined. The variance of alpha-fetoprotein values for serum samples assigned to gestational age categories from 15 to 19 weeks of gestation based on menstrual history and that of samples classified according to sonographic estimates do not show clear-cut differences. However, when an individual sample is referred to a calibration curve of normal values, errors in gestational age estimates significantly increase the possibility of a false positive/false negative decision for high-level percentile classification. The effect of such an error is least at 16 to 17 weeks of gestation.