Fate of fertilized human oocytes.

Establishing the proportion of fertilized oocytes and early human embryos that proceed to term may help policy makers in their evaluation of when the life of a new human individual begins and in determining the nature of protection to be accorded to it. The rate of spontaneous abortions, although increasing with age, overall does not exceed 15%. However, abortion rates refer only to 'clinical pregnancy', whereas early embryonic loss is more common than generally believed. Evidence of such wastage comes from many sources. Human fecundity rarely exceeds 35% and may be decreasing due to deterioration in semen quality. Embryological studies show that 50% of randomly recovered preimplantation embryos have severe anomalies. The study of sensitive markers of pregnancy, such as human chorionic gonadotrophin, indicates early embryo wastage in the order of 50%. Pregnancy wastage may be a function of the time lapse between ovulation and implantation as the implantation window extends between menstrual cycle days 20 and 24. Finally, data obtained with natural IVF cycles also indicate major losses, with an overall pregnancy rate of 7.2% per cycle and 15.8% per transfer. These data, however, are biased by a high cancellation rate and low oocyte retrieval in natural IVF cycles.