Why are multiple pregnancy rates and single embryo transfer rates so different globally, and what do we do about it?

In the early years of in vitro fertilization, overall pregnancy rates were low, and it was considered necessary to transfer more than one embryo to increase the chances of pregnancy. It was not until advances in assisted reproductive technologies resulting in increased pregnancy rates that the concept of transferring just one embryo was considered possible. A consequence of improvements in implantation rates was also an increase in multiple pregnancies when more than one embryo was transferred. Although some countries have reduced the number of embryos transferred, international data show that in many parts of the world high twin and higher order multiple pregnancy rates still exist. Even in developed countries these problems persist depending on clinical practice, funding of health services, and patient demands. Perinatal and other outcomes are significantly worse with twins compared with singleton pregnancies and there is an urgent need to reduce multiple pregnancy rates to at least 10%. This has been achieved in several countries and clinics by introducing single embryo transfer but there are many barriers to the introduction of this technique in most clinics worldwide. We discuss the background to the high multiple rate in assisted reproduction and the factors that contribute to its persistence even in excellent clinics and in high-quality health services. Practices that may promote single embryo transfer are discussed.