Background: The aim of the current review was to determine if single versus two or more embryos, or double versus three or more embryos, transferred to the woman of a subfertile couple at in vitro fertilisation (IVF) maximises the likelihood of pregnancy, while minimising the likelihood of multiple pregnancy and adverse sequelae. Methods: Studies were identified that reported maternal, infant and cost outcomes following embryo transfer at IVF. Results: Three randomised trials and 17 cohort studies were included. From two randomised trials, single embryo transfer was found to result in decreased incidence of clinical pregnancy, multiple pregnancy and low birthweight. In the cohort studies for single embryo transfer compared with transfer of two or more embryos the incidence of live birth and singleton pregnancies was unchanged, and the incidence of multiple pregnancies and low birthweight was reduced. For double embryo transfer compared with the transfer of three or more embryos, the incidence of clinical pregnancy, live birth, preterm birth and low birthweight babies was reduced. Conclusions: Information on neonatal and maternal outcomes following transfer of different numbers of embryos is limited. Transfer of one embryo does not alter the likelihood of a singleton pregnancy or birth when compared to transfer of two or more embryos. Transfer of one or two embryos decreases the risk of a multiple pregnancy, preterm birth and low birthkveight. Further large, well-designed randomised trials are required to provide maternal and neonatal outcomes of relevance to a couple undergoing IVF.
