EMBRYO CRYOPRESERVATION

Just 15 years ago the first pregnancy obtained with a frozen and thawed embryo was reportd in the world. Since then, most in vitro fertilization programs have chosen cryobiology aiming at enhancing pregnancy chances in one only ovarian stimulation cycle. As ovulation induction protocols have augmented the number of recruited healthy oocytes, the responsible for keeping exceeding oocytes has grown. It is not rare nowadays to collect 10, and often more than 10 mature oocytes from a woman. Before freezing techniques were routinely used in a lab, the woman who produced many gametes had a limited number of oocytes to be inseminated or the risk of having healthy pre-embryos discarded, because only three or four pre-embryos could be securely tranferred to the uterus after fertilization.

Recently, it has been demonstrated that pregnancy potential after thawing, in some centers that perform freezing, is almost equal to transfering fresh pre-embryos. Furthermore, these programs report relatively high clinic pregnancy rates, having all frozen embryos exceeded 60%.

Nevertheless, 1995 national and world statistics haven’t reflected comparable results between unfrozen and fresh embryo cycles. Disease control centers in human assisted reproduction with fertility techniques and clinic reports (USA) indicated that, out of the 7465 cycles performed with unfrozen embryos (excluding egg donation), only 15% originated born babies. This unencouraging rate was identical to the one issued by The Society for Technology in Assisted Reproduction, in 1996, and is lower than the 25% babies born by transference, or the 21% babies born per cycle, reported after fresh embryo transfers from the same sources. Moreover, it seems that the French program doesn’t have better results with freezing techniques. One recent report, detailing the embryo cryopreservation data in France in a period of over 9 years showed that just 3,5% frozen embryos transfers originated born babies. World data indicate a clear need to improve cryopreservation technique knowledge.

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