What is the most relevant standard of success in assisted reproduction?: no single outcome measure is satisfactory when evaluating success in assisted reproduction; both twin births and singleton births should be counted as successes.

The 2002 recommendation of the consensus meeting of the European Society of Human Reproduction and Embryology (ESHRE) that the outcome measure of assisted reproductive technology (ART) and non-ART should be 'singleton live birth rate' could profoundly effect the ability of infertility patients to become pregnant. We reviewed published reports and new data concerning elective single embryo transfer (eSET) vs. double embryo transfer (DET) and the outcome of twin pregnancies in the United States, as well as recommendations of other Societies concerning number of embryos to transfer and methods used to measure ART success. We found that no single outcome measure of ART is ideal. Mandatory eSET would result in 42%-70% fewer births compared to DET. Infertility treatments account for only 12% of all twin pregnancies and 4% of all premature births in the United States. Twin and singleton births due to ART do not occur earlier than spontaneously conceived twins and singletons unless they started as triplet and higher order pregnancies. Multiple outcome measures are necessary when evaluating ART success. Twin as well as singleton births should be counted as ART successes. The essential aim of infertility treatment should be a healthy low order (singleton or twin) birth.