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Embryo Freezing

Embryo freezing at a glance

  • Embryo freezing during the IVF (in vitro fertilization) process involves cryopreservation, which is freezing and storing of a fertilized egg to be implanted later for a pregnancy.
  • Embryo freezing is a good option for those wishing to delay pregnancy for personal reasons yet increase the chance of success by using the woman’s more viable eggs when she is younger.
  • Frozen embryos are also a good option for people whose future fertility may be compromised, such as a cancer patient facing fertility harming treatment.
  • Another benefit of embryo freezing is that it allows for multiple embryo implantations in the future from one IVF cycle in which multiple eggs are retrieved and fertilized as embryos.
  • The risks of embryo freezing are the same as those associated with IVF in general, and the freezing of the embryo shows no evidence of it differing in viability, development or quality from other embryos.
woman research embryo freezing process on laptop | Arizona Reproductive Medicine Specialists | Phoenix, AZ

What is embryo freezing by cryopreservation?

Embryo freezing by cryopreservation is the freezing and storage of embryos from an IVF cycle for future implantation and pregnancy. Cryopreservation is the process of cooling and storing eggs, sperm and embryos to a very low temperature to maintain their biologic viability. But the process is slightly different for each.

The cryopreservation process

Embryos are mostly water, which crystalizes as it freezes. Crystallization destroys cells. To avoid crystallization during the embryo freezing process, a cryoprotectant agent (CPA) such as propylene glycerol (PG) or ethylene glycerol (EG) gradually replaces the water within the embryo. There are two methods for freezing embryos, the slow programmable method and vitrification, which is a form of flash freezing.

Slow programmable embryo freezing gradually increases the strength of the CPA while simultaneously cooling the embryo. This takes about two hours to completely freeze the embryo. The vitrification process uses much stronger CPA, resulting in a rapid freezing process. Regardless of the freezing method, the embryos are stored in liquid nitrogen at -321 degrees Fahrenheit (-196.1 Celsius). At that temperature, the frozen embryos presumably can be preserved indefinitely.

While there have been technological advances over the years, embryo freezing has been in use in IVF since the 1980s. Today, frozen embryos have the same success rate in creating viable pregnancies and healthy babies as unfrozen (fresh) embryo transfers in IVF.

What are the steps in using frozen embryos for pregnancy?

Embryo freezing is really just one extra step in the IVF process, which begins with retrieving the woman’s eggs. The eggs are fertilized in the laboratory and the resulting embryo is placed in an incubator.

The process of embryo freezing usually begins an average of five days after fertilization. This allows time for the fertilized egg to incubate and develop into embryonic cell clusters called blastocysts. Though still only about the diameter of a strand of hair at 0.1 to 0.2 mm, an embryo that has developed to the blastocyst stage has a greater chance than less-developed embryos of successfully freezing, thawing and implanting in the uterus for pregnancy.

If parents have concerns about congenital conditions in the child or pregnancy failure that can be caused by genetic flaws in embryos, preimplantation genetic diagnosis (PGD) may be performed. This can be done on the embryos prior to freezing and can also be performed on previously frozen embryos. After PGD, fresh embryos without genetic defects are frozen, using either slow programmable freezing or vitrification, and stored.

When the mother (or gestational carrier if one is being used) is ready to have a child, the embryo is slowly thawed, the CPA is replaced with water, the embryo is examined to verify its quality and then implanted in the womb for a pregnancy. While each patient’s situation and needs differ, fertility specialists seek to implant the minimum number of embryos to achieve pregnancy and minimize the risk of multiple births.

Who should consider frozen embryos for fertility preservation?

  • Couples undergoing IVF treatment can freeze the embryos created during one IVF cycle for multiple, future attempts at pregnancy or additional children. Although the technology is constantly improving, pregnancy rates resulting from frozen embryos remain greater than from using frozen, unfertilized eggs that are thawed and combined with sperm in IVF for an embryo.
  • Women or couples who need to postpone pregnancy due to life events or career demands, but who wish to preserve their fertility while their eggs are young and more likely to result in a pregnancy.
  • Women or couples facing certain medical diagnoses or treatments that harm fertility, such as some cancers, can improve their chances of later pregnancy and childbirth through embryo freezing.

Benefits of embryo freezing for fertility preservation

For couples undergoing fertility treatments, the financial cost and physical demands of an IVF cycle can be economized over the long term through embryo freezing. Maturing and retrieving the eggs is often the most expensive and medically intensive step in the IVF process.

But by freezing the embryos created after retrieval of multiple eggs, a woman or couple can have multiple attempts at future pregnancy from the one IVF cycle, rather than needing another IVF cycle. Other benefits of embryo freezing include:

  • Babies born from frozen embryos do not have a higher incidence of health problems than babies born from basic IVF or conventional conception.
  • Couples with concerns about congenital conditions in a resulting child and the higher rates of pregnancy failure due to genetically flawed embryos can have the embryos genetically tested (PGD). Only embryos free from genetic defects are implanted.
  • Unused frozen embryos can be donated to infertile couples or to medical science for research.

Considerations of using frozen embryos

  • Disposition of unused frozen embryos can be ethically complicated for some couples and individuals. Donation to other infertile couples can sometimes be an option. Donation of embryos for research or discarding them can be difficult choices for some.
  • Custody claims and disposal disagreements over embryos in the event of divorce, remarriage, death or other life events, can be contentious. Couples and individuals should consider seeking the advice of an attorney experienced in such issues, who can explain how the laws apply and establish agreements beforehand.
  • Cost of frozen embryo cryostorage varies widely and can become a burden for some couples over the long term.


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