The chief sources of progesterone in the body are the adrenal, ovary, and placenta.  These are the chief sources of progesterone.  Primarily we are more concerned and we always think about the progesterone, which is originated from the ovary but believe me sometime in the early follicular phase till mid-follicular phase. Really, we are not very sure this progesterone is of adrenal origin or ovarian origin. The physiological level of progesterone in women ranges from 0.23 ng/mL in the follicular phase to 8.3 to 25 ng/mL. The primary role of progesterone is the secretory transformation of the endometrium which is just appropriate for implantation, but for this secretory transformation, estrogenized endometrium is a must.  For the action of progesterone, the estrogenic of the endometrium is a must, and while this secretory transformation, there is a positive shift to Th2 dominance. Apart from that, it increases endometrial vascularity and plays a very important role in calming down the uterus.  It causes uterine relaxation, stabilizes the lysosome membrane.  It blocks the chemokine to reduce the progesterone synthesis and nitric oxide synthesis so these are the different ways how it relaxes the uterus.  On the whole, progesterone helps to make the intrauterine environment conducive for embryo implantation and acceptance.

Huang said that progesterone if it is elevated more than 2 surely it is detrimental to oocytes and even if the embryos formed out of these oocytes are transferred in a frozen embryo transfer cycle next time later on it is less likely to give a pregnancy.  So there is a limit to everything   Borderline cases can be saved by blastocyst transfer but not all.  The embryo quality is probably undisturbed till the progesterone elevation is up to 1.75 or 2 but if it is more than 2, there is a definite affection of the oocyte so the P4 cutoff depends on a lot of things.

If the endometrial thickness is less than 9 mm, okay, 9 to 12 and more than 12, will it not be necessary to have a different requirement of progesterone for doing the secretory transformation is food for thought, is not a question to be answered.  It is food for thought altogether.  We don’t get a pregnancy rate when the ET is more than 14 mm.  When we use 200 mg three times day vaginal progesterone, we have not tried using 300 mg or timed 400 mg for thicker endometrium; it is just food of thought.

You can find out more details by enrolling in Progesterone in ART – friend and enemy by Dr. Rajesh Koradia (MBBS, MD) here, https://docmode.org/progesterone-in-art-friend-and-enemy-dr-rajesh-koradia/