Endometrial preparation for third-party parenting and cryopreserved embryo transfer

The advent of third party parenting ushered in the era of artificial stimulation of the endometrium. Initially intended only for patients with ovarian failure, exogenous induction of endometrial receptivity was quickly shown to be as good as natural endometrial preparation, with the advantage that the timing of embryo transfer could be controlled. It is perhaps surprising that even though the ovary produces a variety of steroids, that estradiol (E₂) and progesterone (P) alone would be needed to achieve optimal receptivity; no other substance has ever been shown to improve on the basic regimen of E₂ and P. A variety of routes of administration are available for both E₂ and P and physiologic (or supraphysiologic) serum or endometrial tissue levels of both can be achieved. The optimal duration of E₂ stimulation and the timing of the onset of P administration continue to be debated, but it appears that imitating the sequence that normally occurs in nature leads to optimal results. The poorly responsive endometrium and cases of recurrent implantation failure remain a challenge, but the clear majority of patients can successfully achieve pregnancy as long as embryos of adequate quality are transferred.