How do fertility treatments specifically increase twin odds?

Fertility treatments increase the odds of having twins primarily in two ways:

1. Ovulation-Stimulating Medications: Drugs like Clomiphene (Clomid), Letrozole (Femara), and injectable gonadotropins (including follicle-stimulating hormone, FSH) stimulate the ovaries to mature and release multiple eggs during a single cycle. The release of multiple eggs increases the chance that more than one egg will be fertilized, leading to fraternal twins.
2. In Vitro Fertilization (IVF) and Embryo Transfer: During IVF, eggs are fertilized outside the body and then embryos are placed into the uterus. Transferring multiple embryos at once raises the odds that more than one will implant successfully, resulting in twins or multiples. In women under 35, about 12% of IVF pregnancies with multiple embryo transfers result in twins. The use of ovarian stimulation in IVF also promotes release and fertilization of multiple eggs.

Other contributing factors in fertility treatments include the woman’s age and the number of embryos transferred. Modern IVF practices often aim to reduce twin pregnancies by transferring a single embryo (single embryo transfer, SET) to minimize risks associated with multiples.

In summary, fertility treatments increase twin odds mainly by causing the ovaries to release multiple eggs simultaneously or by placing multiple embryos in the uterus to improve pregnancy chances.