The purpose of this study was to evaluate fetal outcome and maternal complicat ions of multifetal pregnancy reduction to a single fetus or twins. To evaluate s afety and efficacy of transabdominal chorionic villus sampling for karyotyping b efore fetal reduction. Four hundred twenty-four consecutive multiple pregnancie s were reduced to twins (255 pregnancies) or a single fetus (169 pregnancies) at 8 to 13 weeks of gestation after transabdominal chorionic villus sampling for f etal karyotyping. Fetal and maternal outcome were observed prospectively and com pared with control series of twin (147) and singleton (885)-pregnancies in whic h reduction procedures were not performed. Transabdominal chorionic villus sampl ing was performed successfully in 100%of the cases. The accuracy of karyotyping was 99.2%. The overall pregnancy loss rate after reduction was 3.3%. No diffe rences were observed between study and control series for severe prematurity, lo w birth weight, and neonatal deaths. Mean gestational age at delivery (35.2%vs 38.1%) and mean birth weight (2180 g vs 2873 g) were significantly lower; prete rm delivery (64%vs 11%), neonatal death (3.4%vs 0.6%), and maternal complica tions (42.8%vs 9.5%) were significantly higher when the reduction was to twins rather than in reduction to a single fetus. Pregnancy loss rate did not differ between study series. The overall rate of chromosomal abnormalities in the study series was higher (relative risk, 2.0) than in singleton control series. The ou tcome of multiple pregnancies that were reduced to a single fetus or twins was s imilar to that of nonreduced pregnancies; fetal and maternal complications were significantly lower in the series of pregnancies that were reduced to a single f etus. The safety and efficacy of transabdominal chorionic villus sampling and th e higher pregnancy rate of chromosomal abnormalities in multiple pregnancies imp ly that fetal karyotyping should be advised before fetal reduction. The purpose of this study was to evaluate fetal outcome and maternal complicat ions of multifetal pregnancy reduction to a single fetus or twins. To evaluate s safety and efficacy of transabdominal chorionic villus sampling for karyotyping b efore fetal reduction. Four hundred twenty-four consecutive multiple pregnancie s were reduced to twins (255 pregnancies) or a single fetus (169 pregnancies) at 8 to 13 weeks of gestation after transabdominal chorionic villus sampling for f etal karyotyping. Fetal and maternal outcome were observed prospectively and com pared with control series of twin (147) and singleton (885) -pregnancies in whic h reduction procedures were not performed. Transabdominal chorionic villus sampl ing was performed successfully in 100% of the cases. The accuracy of karyotyping was 99.2%. The overall pregnancy loss rate after reduction was 3.3%. No diffe rences were observed between study and control series for severe prematurity, lo w birth weight, and neonatal deaths. Mean gestational age at delivery (35.2% vs 38.1%) and mean birth weight (2180 vs 2873 g) were significantly lower; preterm rm delivery (64% vs 11%), neonatal death Pregnancy loss rate did not differ between study series. The overall rate of chromosomal abnormalities in the study series was higher (42.8% vs 9.5%) were significantly higher when the reduction was to twins rather than in reduction to a single fetus. Relative risk, 2.0) than in singleton control series. The ou tcome of multiple pregnancies that were reduced to a single fetus or twins was s imilar to that of nonreduced pregnancies; fetal and maternal complications were significantly lower in the series of pregnancies that were reduced to a single f etus. The safety and efficacy of transabdominal chorionic villus sampling and th e higher pregnancy rate of chromosomal abnormalities in multiple pregnancies imp ly that fetal karyotyping should be advised before fetal reduction.