Relative contribution of ovarian stimulation versus in vitro fertilization and intracytoplasmic sperm injection to multifetal pregnancies requiring reduction to twins.

The proportion of twins resulting from multifetal pregnancy reduction of higher-order multiples is increased in pregnancies resulting from hormone stimulation when compared with twins following in vitro fertilization/intracytoplasmic sperm injection treatment. These reduced twin pregnancies may carry a higher perinatal risk compared with other twin pregnancies, which should be taken into account when assessing the perinatal outcome of twin pregnancies after assisted reproduction.