Twin differences in birth weight: the effects of genotype and prenatal environment on neonatal and post-neonatal mortality.

There is much evidence to suggest that both genes and prenatal environment influence life chances. However, recent within-twin estimates also raise questions about how the influence of genes and prenatal environment may vary across different subgroups of a population and over time. This paper explores such potential variation within the 1st year of life. Using data on twin births from the 1995-1997 Matched Multiple Birth Database and an analytic strategy based on the Weinberg assumption, this paper considers how associations between birth weight and infant mortality vary across identical and fraternal twins, gestational age, and time. Results suggest that the influence of genes and prenatal environment vary most significantly by gestational age. In pregnancies that lasted less than 37 weeks, within-twin variation in prenatal environment is able to account for negative associations between birth weight and infant mortality. However, in pregnancies that lasted 37 weeks or longer, underlying genetic variation across fraternal twins appears to be largely responsible for birth weight-mortality associations. Such distinct findings by gestation suggest that genes and prenatal environment may play varying roles in birth weight-mortality associations across different situations.