During the late 1970's Professor Alan Trounson of Monash University in Australia helped pioneer embryo freezing techniques allowing embryos to be frozen then thawed for future use. Today the process of freezing unused embryos is widely performed in IVF throughout Canada and around the world.
When surviving fresh embryos are not utilized during a fresh IVF cycle they are routinely frozen (cryopreserved) in liquid nitrogen (-196C) by the attending clinic. They can be stored for years, usually at a fee charged annually. When ready, embryos can be thawed and then placed into the uterus. The procedure is similar to the transfer of fresh embryos.
One risk to freezing embryos is the potential that some or all will not survive the thaw process. Clinics maintain thaw-rate statistics. The range of success from clinic to clinic may vary between 40-70% on average.
The number of embryos to thaw for a frozen cycle will depend on the number of embryos available, the age of the patient, and the patient's infertility diagnosis. For example, a pre-disposition for miscarriage caused by genetic factors could allow for a higher number of embryos to transfer than a patient without genetic concerns. The goal is to achieve a single pregnancy without accepting excessive risk for multiple pregnancy.
Pregnancy success rates are typically lower with frozen embryos than with fresh embryos, by approximately 10%. In other words, a 30% change of pregnancy success in a fresh cycle might be reduced to 20% with a frozen cycle. Success rates will vary from clinic to clinic and from patient to patient.