Induction of final maturation of oocytes is a procedure that is usually performed as part of controlled ovarian hyperstimulation to render the oocytes fully developed and thereby resulting in optimal pregnancy chances. It is basically a replacement for the luteinizing hormone (LH) surge whose effects include final maturation in natural menstrual cycles. The main medications used for induction of final maturation are human chorionic gonadotropin (hCG) and GnRH agonist. In fresh (rather than frozen) autologous cycles of in vitro fertilization, final oocyte maturation triggering with GnRH agonist instead of hCG decreases the risk of ovarian hyperstimulation syndrome but decreases live birth rate. In cycles followed by oocyte donation, use of GnRH agonists instead of hCG decreases the risk of ovarian hyperstimulation syndrome with no evidence of a difference in live birth rate. Usage with ovulation Induction of final maturation also initiates the mechanisms that eventually result in ovulation, and thereby makes the oocytes destined to undergo ovulation unless artificial oocyte retrieval is performed first. Therefore, induction of final maturation is also called triggering oocyte release from the ovary, and the administration of pharmaceutical drugs to induce final maturation is colloquially called giving a "trigger shot", even if the plan is to perform artificial oocyte retrieval before ovulation. Administration of a drug to trigger oocyte release without oocyte retrieval results in a predictable time of ovulation, with the interval from drug administration to ovulation depending on the type of drug. This avails for sexual intercourse or intrauterine insemination (IUI) to conveniently be scheduled at ovulation, the most likely time to achieve pregnancy. In ovulation induction, using clomifene for intended conception by sexual intercourse, however, triggering oocyte release has been shown to decrease pregnancy chances compared to frequent monitoring with LH surge tests. Therefore, in such cases, triggering oocyte release is best reserved for women who require IUI and in whom LH monitoring proves difficult or unreliable. It may also be used when LH monitoring hasn't shown an LH surge by cycle day 18 (where cycle day 1 is the first day of the preceding menstruation) and there is an ovarian follicle of over 20 mm in size. Usage with artificial oocyte retrieval In in vitro fertilization (IVF), induction of final maturation avails for egg retrieval when the eggs are fully mature. In IVF, final maturation induction is preceded by controlled ovarian hyperstimulation. It is suggested that there should be a size