Monozygotic twins are two individuals derived from one sperm and one egg; dizygotic twins are individuals born together but originating from different eggs fertilized by different sperms. Dizygotic twinning rates vary in different populations but have risen in the last 30 years on account of ovarian stimulation and the increasing tendency to postpone childbearing. Iatrogenic ovarian stimulation induces the simultaneous growth of multiple follicles leading to the ovulation of more than one egg and hence multiple fertilization. Milder forms of ovarian stimulation in intrauterine insemination (IUI) cycles and the choice of single embryo transfer (SET) for in vitro fertilization (IVF) cycles help prevent iatrogenic twinning. The risk of dizygotic twinning linked to postpone childbearing and the higher follicle-stimulating hormone (FSH) concentration poorly controlled by an older ovary is harder to change in the short term. As increasing number of women delay reproduction, for understandable social reasons, the ovarian/pituitary system in women older than 25 generates 5–8 additional twins per 1000 births every additional year of age. A new social strategy to encourage reproduction in young couples would be needed to influence the current widespread tendency.