The incidence of multiple pregnancies has increased considerably over the past 20 years in the western world. The number of twin pregnancies has increased by approximately 25% and the rate of triplet and higher order multiple pregnancies has more than doubled. This is due to the use of ovarian hyperstimulation and assisted reproduction techniques, and to the increasing maternal age. Multiple pregnancy can be considered as the most important adverse outcome of infertility treatment. The main determinant for outcome and management of multiple pregnancies is the chorionicity. This can be determined by ultrasound until 14 weeks of gestation. Twenty percent of all twin pregnancies are monochorionic, and 15% of these develop severe twin to twin transfusion syndrome, which is best treated by LASER separation of the communicating blood vessels. Obstetric complications associated with multiple gestation also occur more frequently in monochorionic twins. They include increased incidence of preterm labor, intrauterine growth restriction and assisted or surgical delivery. Neonatal problems include low birthweight and increased prevalence of congenital malformations. Overall, there has been a decrease in neonatal mortality of twins and triplets over the past 10 years, which is mainly due to the enormous advances in neonatal intensive care.