Is egg freezing ready for prime time?

Key Points

Yes. This technology has arrived and appears to be safe.

Aging eggs chiefly to blame for decline in age-related fertility

However, unlike frozen embryos, which are a standard part of IVF practice and represented by more than 150,000 births worldwide, consistent and acceptable pregnancy rates using frozen eggs have been elusive.⁴ Approximately 150 births have occurred in the 20 years since Chen reported the first birth after oocyte cryopreservation in 1986.⁵

The technical difficulties of oocyte cryopreservation have been related to three things:

1. high water content and intracellular ice crystal formation upon freezing;

2. hardening of the zona pellucida during cryopreservation, and thus difficulty with subsequent fertilization; and

3. the relatively large size of the cell, and thus an unfavorable surface-to-volume ratio for equilibration of solutes.

These roadblocks have been gradually overcome by the use of cryoprotectants, ICSI for fertilization, and the replacement of sodium in freezing media with an osmolyte like choline.⁶ The net effect has been a substantial increase in oocyte survival and viability after cryo-preservation.⁷

A recent meta-analysis by Oktay and colleagues reported a live birth rate per transfer of 28.4% following oocyte cryopreservation, a rate comparable to cryopreserved embryos.⁸ More recently, investigators from several countries have reported pregnancy rates above 35% from cryopreserved oocytes, similar to the 37.5% rate derived from fresh oocytes. Many studies also report multiple gestations following oocyte cryopreservation, suggesting that the technology is robust.

Alternatives to oocyte cryopreservation have many limitations. The current recommendation for single women to cryopreserve embryos derived from donor sperm introduces many personal challenges, such as paternity issues related to nonanonymous sperm donation, the disposition of embryos should that woman marry, and in the case of anonymous sperm donation, the emotional discomfort of not knowing who fathered the child. In addition, frozen embryos are associated with lower pregnancy rates than fresh ones. Other techniques to preserve oocytes using ovarian cortex cryopreservation or in vitro maturation are still in their infancy. There's even been a recent report describing residual cancer cells in ovarian cortex harvested from a patient with leukemia.⁹

Safety questions remain

A number of questions relating to the safety of cryopreserved oocytes remain. The number of established pregnancies and deliveries derived from cryopreserved oocytes is limited. Despite concern that oocyte cryopreservation may induce damage to the meiotic spindle that could lead to chromosomal aneuploidy or other karyotypic abnormalities, no increase in the number of abnormal or stray chromosomes in thawed, previously cryopreserved oocytes has been observed.¹⁰ The incidence of chromosomal abnormalities in human embryos obtained from frozen oocytes was no different from noncryopreserved controls using fluorescence in situ hybridization.¹¹