Dear Fr Bloom,

In re your earlier question, the estimates of an 80% spontaneous "abortion" (although failure to implant isn't really an abortion, but that's a whole different topic) are based on extrapolations from such research as reproduced below in the abstract from BioMedNet. The tabular information at the end doesn't cut and paste well, and I don't know if a non-member can go directly to the abstracts - but membership is free, easy, doesn't generate any spam, and is just as easy to undo (should you wish to visit the site). The search which turned up the below was done on the phrase : human and embryo and survive The extrapolations are (so far as I know) based on the assumption that an implanted embryo is equivalent to a "natural" embryo to an otherwise ready uterus. There is no apparent reason that it shouldn't be, so this is a reasonable assumption.

The abstract: Reprod Nutr Dev • Volume 28 • Issue 6B VIEWING OPTIONS: [MEDLINE], [full MEDLINE], [related records] Please note that the original text is not in English

[Timing of embryo transfer and success of pregnancy in the human] Mandelbaum J, Junca AM, Plachot M, Cohen J, Salat-Baroux J Reprod Nutr Dev 1988 28:6B 1763-71 Vernaclular Title Moment de la transplantation embryonnaire et succès de la gestation chez l'humain. Abstract The best moment for human embryo transfer has not yet been accurately determined. The human uterus is able to receive young embryos (2, 4 cell-stage and even pronucleated eggs), insure their growth and implantation at a rate that does not exceed, however, 15% for one transferred egg. At the present time in vitro culture to the blastocyst stage, which impairs human embryo viability, should be avoided. Contrary to classical IVF, it becomes possible to dissociate embryo and endometrial ages when transferring frozen-thawed eggs. Our study concerns 443 spontaneous, stimulated or artificial cycles, performed in patients with normal or without endogenous ovarian function (tabl. 1). There was trend towards enhanced pregnancy rates (17%) for synchronous as compared to one-day asynchronous transfers (9%) (tabl. 2). Similar data, widely stated in mammals, emphasize the necessity of a precise chronology of embryo transfer. Embryos only survive and get implanted when placed in a receptive uterus. The onset of the refractory period appears to be closely linked to the maternal steroid environment. Consequently, no pregnancies arose from transfer of donated embryos beyond 6 days of progestative supply in women deprived of endogenous ovarian function. In the same way, administration of progesterone 40 h before oocyte recovery seemed to advance the refractory phase. Indeed, a reduction in the pregnancy rate per transfer was observed in such circumstances without any obvious impairment of embryo viability (tabl. 4). The best pregnancy rate was obtained in synchronous transfers while a one-day disynchronization reduced this rate by half.

MeSH Animal, Blastocyst (PH), Embryo Transfer (*), English Abstract, Female, Fertilization in Vitro, Human, Menstrual Cycle, Pregnancy, Time Factors, Uterus (PH) Author Address Service de Gynécologie-Obstétrique, CHI Jean Rostand, Sèvres.

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