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Why do IVF treatments increase the likelihood of 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.

In vitro fertilization (IVF) increases the likelihood of multiple births primarily because of the practice of transferring multiple embryos into the uterus to improve the chances of a successful pregnancy.1

Historically, to maximize the probability of implantation, clinics often transferred several fertilized embryos at once—sometimes as many as four—which significantly raised the risk of multiple fetuses being carried to term simultaneously. While modern advancements have allowed for the use of fewer embryos without sacrificing success rates, many patients still choose to transfer multiple embryos due to the high cost of IVF cycles and the lower success rate associated with single embryo transfer (SET).1

It is important to recognize that multiple births carry significant health risks, including premature birth, low birth weight, and an increased predisposition to developmental and neurological disorders such as cerebral palsy. Consequently, the medical community emphasizes the ethical importance of developing techniques that prioritize the health of both the mother and the children by reducing the necessity for multiple embryo transfers.21