The pivotal role of luteal phase support (LPS) in establishing and maintaining IVF pregnancies has been one of the earliest subjects to become evidence-based in clinical ART. Following controlled ovarian stimulation (COS) and ovulation triggering by hCG, pulsatile pituitary LH secretion has been demonstrated to be severely compromised and unable to support normal function of the corpora lutea, resulting in a deficient luteal phase that must be pharmaceutically supported. This was initially demonstrated for GnRH agonist protocols and subsequently confirmed for GnRH antagonist cycles as well. After implantation, embryonic hCG takes over pituitary LH in supporting the corpus luteum (CL) and maintains its function until the establishment of the luteo-placental shift, at around the 8th gestational week. It has been clearly demonstrated that LPS is crucial in filling the gap between the disappearance of exogenously administered hCG for ovulation triggering and the initiation of secretion of endogenous hCG from the implanting conceptus. Early studies have estimated that exogenously administered hCG remains in the circulation for up to 7 days, and that the CL has a remarkable ability to recover after a week of deprivation from gonadotropin stimulation (1).