We compared mean birth weight in grams and gestational age in days between infants of same birth order in families with and without birth defects. Gestational age data were screened for birth weight – gestational age consistency using a method based on the assumption of normally distributed birth weights for each gestational week (Skjaerven R et al. New standards for birth weight by gestational age using family data. Am J Obstet Gynecol 2000; 183: 689-96).