In this paper, we study the socio-economic determinants of birth weight, with a focus on the mother's family status. We use Austrian birth register data covering all births between 1984 and 2007 and find that a mother's marriage is associated with a higher birth weight of the newborn, in the range of 40 to 60 g. The significant impact is retained if we include mother fixed effects or use an instrumental variable approach to account for unobserved mother heterogeneity. However, the magnitude of the causal effect (37 g) clearly indicates the importance of selection into marriage. Divorce around pregnancy results in significantly lower birth weights than the birth weights of babies born to single mothers. Family status effects in the 2000s are stronger than they were in the 1980s, and quantile regressions suggest that family effects are more pronounced at the lower quantiles of the birth weight distribution and less pronounced at higher quantiles. We conclude that the life situation of expectant mothers has an important influence on the birth weight of newborns, especially at the lower tail of the birth weight distribution. Copyright © 2013 John Wiley & Sons, Ltd.