The effect of maternal weight on maternal serum alpha-fetoprotein levels.

In 902 singleton pregnancies, maternal serum alpha-fetoprotein (AFP) levels between 15 and 20 weeks of pregnancy were significantly related to maternal weight (r = 0.24, p less than 0.0001). Lighter women had on average higher AFP levels than heavier women, perhaps on account of the greater concentration of AFP in their relatively smaller volume of blood. The mean AFP level for women weighing less than 45 kg was 68% higher than the mean level for women weighing 85 kg or more. Maternal weight was found to be an important factor which could account for false positive AFP results in antenatal screening for open neural tube defects. A policy of adjusting maternal serum AFP values according to maternal weight among women with borderline positive results could reduce the number having a diagnostic amniocentesis with only a negligible loss of detection for open spina bifida. With such a policy, using a cut-off level of 2.5 x normal median, the false positive rate in the 902 women screened would have been reduced fom 2.8% to 2.0%.