When patients undergo in vitro fertilization cycle, they will produce a number of viable embryos. Any embryos that are not transferred back to the patient they are frozen and stored in liquid nitrogen at -196˚ C until they are needed for later transfer. Freezing embryos in multiple straws also increases chances of getting pregnant in various cycles without undergoing ovarian stimulation and egg retrieval procedure. Embryo freezing is done to avoid OHSS, which would worsen after pregnancy. In addition, embryo freezing is also done to preserve future fertility, for the patients who are undergoing hormone therapy, cancer therapy or any other medical intervention that affects their fertility. Recent evidence suggests that frozen embryo transfer give better pregnancy rates than fresh embryo transfers.
Embryo freezing is done by a technique called ‘Vitrification’ in which rapid cooling and thawing of embryos is done by using special kits. Best quality embryos are selected, placed in solutions according to the protocol, loaded on previously labeled straws and kept in liquid nitrogen tanks with proper labelling and record keeping until future utilization. When required, particular straw is removed from the nitrogen tank, thawing is done by placing the embryos in special thawing solutions, and then kept in culture media inside the incubator until transfer to uterus of patient.
