Ovulation occurs within 38-42 hours of initiation of the spontaneous luteinizing hormone (LH) surge (which can be detected in the blood or urine prior to this event) and/or hCG administered following controlled ovarian stimulation (COS) with gonadotropins. One or more eggs are released with spontaneous or induced ovulation. Those follicles that ovulate and many of those emptied at egg retrieval, then undergo “luteinization”, converting to one or more a yellow bodies or corpora lutea (CL) that produces both progesterone and estrogen. The greater the original number of mature follicles, the greater the progesterone/estrogen production is likely to be. Accordingly, women on fertility drugs have higher luteal phase progesterone/estrogen levels. The effect of the pre-ovulatory hCG injection is usually sustained for 1-2 weeks exerting a protracted influence on ovarian progesterone/estrogen production. A few days later, provided that embryo implantation takes place, the early trophoblast (root system of the conceptus) begins to produce its own progesterone/estrogen as well as hCG, in ever increasing amounts. By the 8th week of pregnancy the early placenta provides for all hormonal needs of the developing conceptus. There is compelling evidence to show that hCG augments ovarian (corpus luteum) progesterone release while also promoting growth and development of the trophoblastic “root system” of the conceptus (which eventually will develop into the placenta) as well as estrogen and progesterone production. Since, at the same time, hCG probably also promotes the production of more hCG, it might be considered to be a self-propagating hormone. By the 8th-9th week of pregnancy, the trophoblast has replaced the ovaries as the dominant source of progesterone and estrogen production. Thereafter there is probably little or no benefit in supplementation with progesterone/estrogen It follows that a low blood progesterone blood level is much more likely to be the consequence rather than the cause of a failing pregnancy. Thus, in such cases the administration of progesterone/estrogen in an attempt rescue a failing pregnancy is tantamount to “shutting the gate after the horse has left the stable.” An obvious situation where progesterone/estrogen supplementation is required is in cases where the woman is an embryo recipient (i.e., ovum donation, embryo adoption, gestational surrogacy and frozen embryo transfers-FET). By the 8th to 10th week of pregnancy, conversion from reliance upon the corpus luteum to sustain the pregnancy has occurred and further fetal development, supported by the hormonal production of the placental trophoblast. Thus there is in my opinion little or no benefit in estrogen/progesterone supplementation beyond the 10th week. While progesterone /estrogen supplementation likely has benefit in cycles involving pituitary down-regulation with GnRH agonists (e.g. Lupron, Buserelin, Superfact, Decapeptyl) or antagonist (Ganirelix, Orgalutron, Cetrotide) where luteal phase hormonal deficiency is more prevalent, there is no conclusive evidence that patients undergoing gonadotropin stimulation without the use of a GnRH agonist or an antagonist would derive benefit from such hormonal supplementation. Hormonal supplementation usually involves the daily intramuscular administration of progesterone +/- vaginal suppositories (comprising estradiol and micronized progesterone) until a blood pregnancy test is performed approximately eight days later (the chemical diagnosis of pregnancy). If the pregnancy test is negative or the plasma hCG levels fails to rise appropriately in the ensuing days, such hormonal support is discontinued. For those that cannot tolerate daily intramuscular progesterone, Crinone-8% or Endometrin vaginal applications can be used instead.