Assisted reproductive technologies (ARTs) aim to increase a woman's chances of becoming pregnant by bringing many female and male gametes into close proximity. Techniques to achieve this objective include ovarian hyperstimulation by maturation of several oocytes, intrauterine insemination (IUI) of concentrated sperm, or in-vitro fertilisation (IVF) by bringing gametes together outside the female body. The very nature of ovarian hyperstimulation--with or without IUI--enhances the risk of multiple pregnancy (eg, two or more babies). In most IVF cycles, more than one embryo is transferred, again resulting in an increased chance of multiple pregnancy. Developed societies have witnessed a large rise in prevalence of twin, triplet, and higher order multiple births, mainly resulting from ARTs. The primary aim of this Review is to increase awareness of the many implications of the present iatrogenic epidemic of multiple births. The background of ovarian hyperstimulation, trends supporting current practice, and strategies to reduce the chance of multiple pregnancy are highlighted.