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Why can fertility drugs increase the chance of twins or triplets?

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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 twins or triplets primarily because they stimulate the ovaries to produce and release multiple mature eggs during a single cycle, rather than the single egg typically released in a natural cycle.1

These medications, particularly directly acting agents like gonadotropins, exert powerful stimulatory effects on the ovaries. By increasing the number of eggs available for fertilization, the probability that more than one egg will be fertilized and successfully implanted in the uterus rises significantly.12

While these treatments have enabled many to conceive, they are associated with an increased risk of multiple births. It is important to recognize that multiple pregnancies carry substantial health risks; infants from such births are often born prematurely and at very low birth weights, which can lead to long-term physical, developmental, and neurological challenges.2