To assess the association with birthweight of prenatal medical care, length of gestation, and other prenatal factors, birth certificate data were studied for babies born in 1978 to mothers who were residents of Alameda or Contra Costa counties, California. Using multiple regression data analytic techniques, adequate prenatal care (defined by the number of prenatal care visits compared to length of gestation and month of start of care) was found to be associated with an increase of 197 grams in average birthweight. This effect was even greater for Black infants and infants of short length of gestation. Adding length of gestation to the equation increased significantly the proportion of the variance in birthweight accounted for. For babies of short gestation (less than or equal to 280 days), the addition of length of gestation was associated with a halving of the association of prenatal care with birthweight. The results suggest that researchers need to take into account the nonlinear relationship between length of gestation and birthweight when assessing factors that affect birthweight.