Human Embryo Freezing in an In Vitro Fertilization and Embryo Transfer Program

Abstract

It is now indisputable that freezing is the preferred way to preserve human embryos obtained after in vitro fertilization (IVF) which cannot undergo embryo transfer (ET) because of obstetrical considerations. The only alternative to avoid the destruction of such embryos may be ET into the uterus of other sterile patients with the approval of the donor couple. Even in this situation embryo freezing introduces further psychologic and ethical considerations, on the other hand it helps to synchronize the age of the embryo with the physiologic status of the recipient uterus. Optimal conditions for embryo freezing in an IVF program may be obtained by the use of a nontoxic cryoprotectant which could be also suitable for very early human embryos. Although high pregnancy rates have been reported after transfer of frozen-thawed human embryos [1, 2], the rate of survival upon thawing appeared inversely related to the duration of in vitro culture: there were only about 50% or 15% transferrable embryos per fertilized oocyte when embryo freezing was done 2–3 days [1] or 4–6 days [2] after in vitro insemination, respectively. Propanediol has been demonstrated to be convenient for the preservation of very early embryos obtained from the mouse, rabbit and cow [3], and humans [4]. The use of this cryoprotectant enables us to freeze fertilized eggs at the normal time for ET, i.e., or 2 days after in vitro insemination. Pregnancies were obtained after transfer of frozen and thawed one-cell embryos [5].