What is Embryo Freezing?
Women undergoing in vitro fertilization (IVF) may have an excess of embryos, or fertilized eggs, that were not implanted at the end of the procedure. In such cases, embryos can be frozen and then transferred to the woman’s uterus at a later time, should she decide to attempt additional pregnancies. In some cases, the additional embryos are donated and transferred to another woman’s uterus.
Embryo freezing, also called cryopreservation, allows multiple embryo transfers from a single egg collection and improves the chances of a live birth. Approximately 50 percent of women may have spare embryos that can be frozen. Cryopreservation has helped thousands of infertile couples to have healthy babies since the 1980s. In fact, the first frozen embryo baby was born in 1984.
Who is a candidate?
Candidates for cryopreservation and the use of future frozen embryos:
- Couples who undergo IVF and have additional embryos they’d like to use later. This gives couples several additional opportunities at pregnancy without having to go through another IVF process involving hormone injections, surgical procedures, numerous doctor’s visits, and the emotional and financial costs.
- Couples in which one partner must undergo treatment for cancer, including chemotherapy or radiation. These treatments can destroy a person’s ability to produce healthy eggs or sperm.
- Single men and women facing cancer treatment that may render them infertile. They can preserve their sperm or eggs (gametes) and use donor eggs to produce embryos to be used after recovery.
- Infertile couples and single women who can use donor embryos to become pregnant.
- Women who are at a high risk of developing severe ovarian hyperstimulation syndrome following ovarian stimulation for IVF.
- Women at risk of embryo implantation being compromised because of endometrial polyps, poor endometrial development, or break through bleeding near the time of embryo transfer or illness.
- Cases in which there was difficulty at the time of fresh embryo transfer, e.g. cervical stenosis (inability to pass through the cervical canal because the cervix is narrowed or scarred).
For emotional, religious or ethical reasons, some couples who undergo IVF treatment and have an excess of embryos choose not to discard them. It’s common for couples to donate their embryos to infertile couples or for research. Most frozen donor embryos come from couples undergoing IVF treatment.
What is the process for embryo freezing?
Embryos can be frozen at any stage – from one to five days after fertilization – as long as they are of good quality. Typically, they are stored in batches of one or more depending on the number of embryos that are likely to be transferred into the woman’s uterus.
Embryos are mixed with a cryoprotectant solution during the freezing process to protect them. This solution prevents ice crystals, which can destroy embryos, from forming between the cells. The embryos are then placed in glass vials and stored in liquid nitrogen freezers at a very low temperature. They are cooled slowly to -196° C (-400°F) using a specialized machine. When needed, the embryos are thawed, and bathed in solutions to remove the freezing cryoprotectants.
What are the success rates?
Frozen embryos can stay deeply frozen for about 10 years and still remain viable. However, many do not survive the freezing and thawing process. The embryos that do survive are often not as healthy as fresh embryos, and the pregnancy success rate with frozen embryos is lower than that of fresh embryos. In 2007, 31.7 percent of frozen embryo transfers resulted in a live birth.