Intrauterine insemination (IUI) and in vitro fertilization (IVF) differ mainly in how fertilization occurs. In IUI, sperm is directly placed into the uterus in order to increase the chances of natural fertilization. IVF, on the other hand, involves retrieving eggs and fertilizing them in a lab before transferring the embryo into the uterus. IVF is generally used for more severe infertility issues or when IUI isn’t effective.