OBJECTIVETo evaluate whether abnormal umbilical cord insertion (UCI) into the placenta is a risk factor for birth weight discordancy in twin gestations.METHODSPathology records of all liveborn twins delivered between January 1993 and June 1996 were reviewed. The information collected included gestational age at delivery, birth weight, gross placental pathology, and placental UCI—velamentous, marginal, or disc. Discordancy in birth weight was defined as an intrapair difference of at least 20%. Analyses were stratified on placental chorionicity. Odds ratios and 95% confidence intervals for birth weight discordancy were calculated based on the presence of an abnormal (velamentous or marginal) placental UCI relative to normal (disc) UCI on both placentae, after adjusting for potential confounders.RESULTSThere were 447 twin pairs identified. Dichorionic diamniotic placentation was present in 358 pairs (80.1%), monochorionic diamniotic in 84 (18.8%), and monochorionic monoamniotic in five (1.1%). There was a 13-fold increase in the risk of birth weight discordancy in monochorionic diamniotic twins in the presence of a velamentous UCI (odds ratio 13.5, 95% confidence interval 1.4, 138.4), with a rate of birth weight discordancy of 46%. This relationship was not demonstrated in dichorionic diamniotic twins (odds ratio 1.0, 95% confidence interval 0.3, 3.5).CONCLUSIONBirth weight discordancy in twins is a different entity depending on chorionicity. The substantial increase in birth weight discordancy in monochorionic diamniotic twins that accompanies velamentous UCI underscores the need for prenatal detection and increased surveillance in these twins.