Single embryo transfer is advocated as a strategy to reduce the frequency of multiple births after in vitro fertilization. Transfer of a single, blastocyst stage embryo increases the likelihood of pregnancy and delivery as compared with transfer of a single, cleavage-stage embryo in women under 36 years of age undergoing their first or second trial of in vitro fertilization. The reason for the higher success rate with blactocysts might be mainly related to an embryo selection process. In the near future, this approach might be appropriate for women older than 36 years. A potential drawback to the use of blastocysts is failure to transfer any embryos. Additional concerns are the possibilities of an increased risk of monozygotic twins and an altered sex ratio of births.