Sept. 28, 2004 -- Although babies born using frozen eggs may grab the headlines and offer hope to women attempting to preserve their fertility, researchers say there is still plenty of room for improvement with this still-evolving technique.
A new Italian study shows the process of freezing and thawing eggs, called oocyte cryopreservation, is a delicate one wrought with potential complications.
Researchers found only 37% of the eggs survived freezing and subsequent thawing, and less than half of those were successfully fertilized.
Overall, out of the 737 frozen and thawed eggs taken from 68 women, only 13 children were born to previously infertile couples. Of these, two pregnancies resulted in twins and all were healthy with no malformations.
The results show that egg preservation methods produce much lower pregnancy success rates than other fertility preservation methods, such as freezing fertilized embryos (an already joined egg and sperm). But oocyte preservation may be a viable alternative to embryo storage for some women who are at risk of becoming infertile due to certain cancer treatments or who otherwise wish to preserve their fertility.
Researchers say eggs harvested from a woman's ovaries are extremely fragile and must be carefully frozen and thawed to prevent damage to the egg. Survival of frozen eggs has historically been very poor because of damage caused by the freezing technique. There has also been some concern over the level of damage that might occur to chromosomes during the freezing process.
In contrast, frozen embryos are more stable and less likely to become damaged during the freezing and thawing process.
But surviving the freezing technique is only part of the process. Once the egg survives freezing and thawing, it is then fertilized using intracytoplasmic sperm injection (ICSI).
The next step is implanting the fertilized embryo into the woman's uterus. In this study, 16% of the thawed and then fertilized eggs were successfully implanted.
Researchers say their results confirm previous studies that showed limited success in achieving pregnancy using frozen human eggs.
"For oocyte [egg] freezing to become an established option, developing methods to obtain success rates similar to embryo freezing is essential," write researcher Andrea Borini, MD, of Tecnobios Procreazione in Bologna, Italy, and colleagues.
In their fertility center, researchers say implantation success rates using frozen embryos were about twice as high as those using frozen eggs during the study period.