It doesnt have to. Ivf, as opposed to using fertility medications with/without artificial insemination, actually gives you a lot of control over your risk of multiple pregnancy, since no matter how many embryos are made in the lab, you can always choose to transfer just one embryo into the uterus (there would still be about a 1% chance of identical twins if the embryo splits).
Multiple Reasons. Fertility treatment does not always lead to multiple pregnancies. Some patients and some doctors are more aggressive with treatment which can increase this risk. Some treatments make more than 1 egg and this risk is increased.
See below. It is because most of ivf clinic prefer to transfer two or more embryos. It is true, with transfer more embryos will increase the pregnancy rate, but it significantly increased the multiple gestation. To reduce the risk of multiple gestation, we have to promote for the single embryo transfer.