Egg freezing (or oocyte cryopreservation) is the freezing of eggs (oocytes) to preserve the fertility of female cancer patients before they undergo chemotherapy or radiotherapy. Egg freezing may be an alternative to embryo freezing in order to avoid the immediate need for a fertilizing sperm in cases where a male partner or sperm donation are not available or acceptable to the patient. The typical egg freezing procedure includes the use of injectable hormones to stimulate the ovaries to promote the growth of multiple mature eggs, a surgical procedure to collect oocytes from the ovary, rapid freezing of mature oocytes, and storage of cryopreserved oocytes for a period of time (e.g. years). In the future, these oocytes can be thawed and used for in vitro fertilization by sperm to create embryos, which can then be transferred into the uterus to achieve pregnancy. Egg freezing is now considered to be an established technology by the American Society of Reproductive Medicine due to significant improvements in pregnancy rates.