Maternal serum alpha-fetoprotein and birth weight.

Maternal serum alpha-fetoprotein (AFP) levels between 16 and 18 weeks of pregnancy were studied in relation to birth weight among 4198 singleton pregnancies resulting in a liveborn or stillborn infant without a neural tube defect. The highest mean birth weight (3365 g) was found among infants born to women with AFP values in the range 0.50 to 0.74 times the normal median for the relevant week of gestation. As the AFP decreased from this level to less than 0.25 times the median, the birth weight decreased by 193 g and the proportion of infants weighing 2500 g or less increased from 4 per cent to 11 per cent. As the AFP level increased from 0.50 to 0.74 times the median to 3.0 times the median or greater, the birth weight decreased by 371 g and the proportion of infants weighing 2500 g or less increased from 4 to 22 per cent. The negative association was the more important of the two since the magnitude was greater and it affected 92 per cent of pregnancies. The positive association among women with low ARP levels was substantially reduced when the analysis was restricted to women who had spontaneous labours. It seems that this minor positive association arises mainly because women in whom gestation is over-estimated tend to have both a low AFP and, later, an apparently prolonged pregnancy leading to the elective delivery of an infant who is, in fact, born early.