Time, time, time: see what governs the luteal phase endocrinology.

Two modes of ovulation trigger are used in IVF: hCG, acting on ovarian LH receptors, and GnRH agonist, eliciting pituitary LH and FSH surges. These two modes are evaluated herein, focusing on how they serve specific time-sensitive events crucial for achieving embryo implantation and pregnancy. hCG trigger is associated with significant timing deviation from physiology. Peak progesterone is not synchronized with implantation window; progesterone level does not rise continuously to a mid-luteal peak, but rather drops from a too early peak. The luteal phase endocrinology post GnRH agonist trigger is characterized by a quick and irreversible luteolysis. Therefore, freeze all strategy is advised, if there is a risk of ovarian hyperstimulation syndrome. If fresh transfer is desired, numerous approaches for luteal phase support have been suggested. However, a thorough understanding of time-sensitive events suggests that a single 1,500 IU hCG dose, administered 48 h post oocyte retrieval, is all that is needed to fully support the luteal phase and secure best chances of achieving pregnancy.