Data were collected on the birth weights of 1,694 offspring of 385 sets of twins including 108 male and 131 female monozygotic pairs. To resolve the influence of birth order from the genetic, environmental, and maternal effects on birth weight, we analyzed the full-sib and maternal and paternal half-sib correlation matrices for birth orders one to five using a causal model that assumed each live-born child had an influence on the weight of the subsequent birth. Prenatal maternal influences explained 40% of the variation in birth weight of the first-born child and 52% for the fifth child; genetic or environmental factors common to monozygotic twins accounted for 72% of this effect, while environmental variables unique to individual mothers were responsible for the remaining 28%. The inclusion of a birth-order parameter resulted in a highly significant improvement in the goodness of fit of the causal model such that by the fifth child, 46% of the maternal variation could be attributed to the cumulative effects of previous live births.