Statistical analysis Statistical analysis will be performed using the SPSS statistical package adjusting for the stratified design. The analyses will be done on the intent-to-treat basis. Categorical data like maternal characteristics and the rates of neonatal morbidity will be compared using Chi square when sample sizes support the approximation. Otherwise, categorical data will be analyzed with two-tailed Fisher exact test if the expected cell frequencies were small. Continuous variables will be compared by Student t test if assumptions of normality and homogeneity of variances appeared to be reasonable or the Wilcoxon rank-sum test. Unpaired variables and differences in distributions will be compared using the Mann-Whitney test. Neonatal outcomes will be analyzed on the assumption that if a neonatal outcome variable occurs in at least one of the fetuses, the pregnancy will be considered affected for that variable. A p-value <0.05 will be considered statistically significant.