• For patients who want to use their frozen embryos from the last treatment (the last treatment can be either successful or unsuccessful)
• For patients whose embryos are all frozen and no fresh transfer is made due to the hyperstimulation risk or due to the reason that the uterus is not suitable for transfer,
• Clinical preference,
• For patients for whom embryos are collected (embryo pool) due to various reasons including low egg reserve etc. after embryos are exposed to genetic testing

Developing eggs and egg retrieval procedure during fresh embryo transfer and hormones released by the ovaries prepare the uterus for embryo transfer. The patient can be prepared with ovulation treatment during frozen embryo transfer despite being not very practical however a better option is to suppress ovulation with medications and to prepare the uterus with oestrogen and progesterone hormone supplements so that the ages of uterus and embryos are the same. Embryos that are thawed on the same day are planted in the uterus prepared as described above.

In vitro fertilization treatment can result in transfer of embryos both fresh and frozen. Some clinics do not have fresh embryo transfer option and they only offer frozen embryo transfer. In our clinic both fresh and frozen embryo transfers are done successfully depending on the patient's condition. The goal in frozen embryo transfer is to prepare the uterus to accommodate pregnancy and to equalize the age of embryos that are thawed and ready to be transferred and the age of uterus. Because theoretically an embryo that is frozen on day 5 can only be accepted by an uterus that is exposed to progesterone for the same number of days to achieve successful pregnancy. Embryos are frozen with a technique called vitrification with very high post-thaw survival rates and then stored in numbered compartments at -196 C in liquid nitrogen containers. These numbers are called embryo addresses and each embryo address is kept in files. In theory there is no upper time limit for storage. We achieved successful pregnancies followed by successful births with embryos that were frozen 11 years ago.