Background Even though twin birthweight discordance is considered to be an important determinant factor for poor perinatal outcome, the level of discordance that justifies delivery is not yet established. The aim of this study is to ascertain the nature of the relationship between birthweight discordance and adverse perinatal outcome in twin pregnancy.

Methodology A retrospective study of all twin pregnancy births of known chorionicity from a regional of 9 hospitals over ten year period. Birthweight discordance was related to adverse perinatal outcome obtained from a mandatory national register (CMACE). Birthweight discordance cut-offs to predict adverse perinatal outcome were evaluated through ROC curves. Logistic regression analysis was performed to evaluate the importance of chorionicity and birthweight discordance in determining adverse perinatal outcome.

Results A total of 2972 twin pregnancies (543 monochorionic and 2429 dichorionic) were delivered after 26 weeks' gestation between 2000 and 2009. Birthweight discordance had an area under ROC curve of 0.71 (95% CI = 0.63 to 0.79) in the prediction of stillbirth and early/late neonatal death. Although birthweight discordance was more prevalent in monochorionic twins, chorionicity itself did not significantly influence adverse perinatal outcome after 26 weeks gestation. Birthweight discordance of 25% occurred in 10% of pregnancies with a 45% sensitivity (95% CI 0.32 to 0.58) for stillbirth and neonatal death.

Conclusion Birthweight discordance pays an important role in determining the outcome of twin pregnancies irrespective of the chorionicity. The data supports the use of a 25% birthweight discordance cut-off for the prediction of perinatal and neonatal death.