Human Chorionic Gonadotropin (hCG) is an injectable hormone, naturally released by a newly implanted embryo. The effects of hCG inside the body are similar to those produced by luteinizing hormone (LH).

Although natural ovulation is induced by luteinizing hormone (LH) peaking (reaching its highest level), hormonal stimulation protocols use hCG instead of LH. The reason is that hCG has a stronger and longer lasting effect, because it does not get cleared from the body as quickly as LH. As well as inducing ovulation, hCG also helps maintain the thick lining of the uterus, which is required for embryo implantation and pregnancy.