The best of the embryos obtained by in vitro fertilization or microinjection is selected for transfer back to the uterus. The other embryos of high quality may be frozen. The frozen embryos may later be thawed and used for transferring them to the uterus. Transfer of frozen and thawed embryos may be performed in connection with a natural menstrual cycle or a hormone cycle, where the inner lining of the uterus matures with the use of mild hormone treatment. The embryo is usually thawed on the day before the transfer. With this it is possible to check that the embryo is viable, and that the cells are beginning to divide.
More than half of the frozen embryos make it through the thawing and the likelihood of pregnancy is 20–40%. A separate annual agreement is signed on freezing and storage of embryos; an annual fee is charged for embryo storage. If you no longer wish to keep the frozen embryos, you may choose to have them destroyed, to donate them to another childless couple, or to donate them to the Clinic, which will use them for quality control and training purposes. Discontinuation of embryo storage is agreed on in writing. The embryos may be kept frozen for years, but embryos originating from donated reproductive cells are kept for a maximum of 15 years after the donation.