Survival and pregnancy outcome after ultrarapid freezing of human embryos.

Contrary to the belief that rapid cooling and thawing of mammalian embryos is detrimental to survival, it has been shown that under certain conditions mammalian embryos can survive rapid freezing-thawing. In this study, 237 human fertilized oocytes (93 pronucleates, 20 multipronucleates, and 124 cleaved embryos) were frozen for a period of 1 day to 7 months using the ultrarapid freezing system. After thawing, 94% of the embryos in the pronucleate group and 89.5% in the multipronucleate group showed normal morphological features, and 79% and 71%, respectively, started to cleave. Forty cleaved embryos were also frozen-thawed, but only 15 (37.5%) survived at thawing. Thirty-four frozen-thawed cleaved embryos were transferred to 20 patients during spontaneous cycles. Four patients became pregnant.