PURPOSE: to define the characteristics of unviable embryos that may be donated for stem-cell research. METHODS: a retrospective evaluation of in vitro fertilzation cycles between January 1995 and January 2005 was structured. Cycles were chosen in which the embryos transferred to the uterine cavity had the same morphological characteristics. Subsequently, the rates of pregnancy, implantation, and involution of the gestational sacs of the fresh embryos as well as of those cryopreserved were analyzed and distributed into groups according to their morphology. Embryos that were symmetric and with 0% of fragmentation were designated type A; asymmetric with up to 25% of fragmentation were designated type B; between 25 and 50% of volume occupied were designated type C, and those with 50% or more of fragmentation were designated type D. RESULTS: one hundred and seventy-two type D embryos transferred in 87 cycles presented low rates of implantation (11%) with 50% of those implanted persisting in development. Embryos with the same morphology, after cryopreservation and thawing, did not show the capacity to evolve. In 36 cycles, 113 thawed type D embryos were transferred, resulting in only one implantation, presenting a minute 3% pregnancy rate. The implanted gestational sac did not evolve, showing a 100% rate of involution. CONCLUSION: embryos with low morphological scores cannot be considered unviable because they are capable, even though with a very low frequency, of supporting gestation. However, these same embryos, after cryopreservation, thawing and transfer showed an insignificant rate of pregnancy, that did not result in viable pregnancy. Therefore, when in excess to requirements, type D embryos should not be cryopreserved; instead, rather than discarded, they should be donated for embryo stem-cell research.
Stem cells; Embryo; Fertilization in vitro; Morphology; Cryopreservation
