The etiology of gastroschisis remains elusive. A classic twin study was used to assess the relative contribution of environmental and genetic factors in its development. Screening of 4872 twin pregnancies identified three unreported twin pairs comprising two monozygous and one dizygous discordant pair of twins. Review of the literature identified an additional 21 twin pairs. We observed lower pair- and proband-wise concordance rates for monozygotic compared to dizygotic twin pairs, pair- and proband-wise concordance ratios below 1.0. Our results suggest environmental to play a greater role than genetic factors in the development of gastroschisis.