Birth weight in triplets is, on average, lower than in singletons and twins, and more children are classified as having very low or extremely low birth weight. Still, there is limited research on factors that affect triplet birth weight, and samples under study are often small. Chorionicity and zygosity influence triplet birth weight, but it is unknown whether the effect of zygosity can be entirely ascribed to the effect of chorionicity or whether zygosity has an additional effect on triplet birth weight. This question was investigated in 346 triplets (from 116 trios) registered with the Netherlands Twin Register for whom data on chorionicity were available. 'Triplet' refers to one child and the set of three triplets is referred to as 'trio'. Trios and triplets were classified based on zygosity and chorionicity. With regression analysis, the effects of zygosity and chorionicity on triplet birth weight were examined, while controlling for gestational age, sex, and maternal smoking during pregnancy. In addition, within the dizygotic trios a within-family comparison was made between the birth weight of the triplets that were part of a monozygotic pair (with some pairs sharing a chorion), and the birth weight of the dizygotic triplet. Based on the classification on individual level, monozygotic, monochorionic triplets had a lower mean birth weight than dizygotic, dichorionic triplets. Most remarkably, in dizygotic trios, monozygotic pairs only had a lower mean birth weight than their dizygotic sibling triplet when the pair shared a chorion. We conclude that having shared a chorion, rather than being monozygotic, increases the risk of a low birth weight. © 2012 Cambridge University Press.