['Our objective was to determine if the low birth weight associated with unexplained elevations of midtrimester maternal serum alpha-fetoprotein levels is due to prematurity or to fetal growth retardation.', 'Rates of preterm delivery and fetal growth retardation were analyzed according to incremental maternal serum alpha-fetoprotein levels in 5555 women, predominantly white, who were screened for neural tube defects (group 1) and 843 women, predominantly black, with risk factors for low birth weight (group 2). Statistical methods included chi 2, t tests, analysis of variance, and regression analysis.', 'In both groups increasing levels of maternal serum alpha-fetoprotein are significantly associated with preterm delivery but not with fetal growth retardation. The preterm delivery rate increased in each group from 8% at levels less than 0.5 multiples of the median to 18.1% (p less than 0.001) at levels greater than or equal to 2.5 multiples of the median in group 1 and 28.1% (p = 0.01) in group 2.'] Are elevated levels of midtrimester maternal serum alpha-fetoprotein associated with preterm delivery but not with fetal growth retardation?