Why can fertility drugs increase the chance of twins or other multiple births?

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Diagram showing the female reproductive system with labeled ovaries, fallopian tubes, uterus, and vagina, alongside hormone pathways and the process from ovulation to fertilization and early embryo development. — infertility Major structures and hormones involved in the initiation of pregnancy. Also seen, at right, is the development of an egg cell (ovum) from follicle to embryo.

Fertility drugs increase the likelihood of multiple births because they are designed to stimulate the ovaries to produce and release eggs more effectively than in a natural cycle.1

Specifically, drugs such as gonadotropins—which include follicle-stimulating hormone (FSH) and luteinizing hormone—are administered by injection to provide a powerful stimulatory effect. This process often causes the ovaries to mature and release multiple eggs simultaneously, rather than the single egg typically produced during a standard ovulation cycle.1

Centrally acting medications, such as clomiphene and aromatase inhibitors like letrozole, also influence this process by signaling the brain to increase the production of FSH. By boosting these hormonal signals, these treatments enhance the recruitment and development of ovarian follicles, which can lead to the release of more than one egg and a subsequent increase in the chance of twins or higher-order multiples.1