According to a study published in JAMA, women who underwent in vitro fertilization (IVF) using frozen eggs were 7 percent less likely to have their pregnancy result in a live birth, as compared to women using fresh eggs.

The Center for Human Reproduction, an NYC-based fertility center founded in 1981, led the study, analyzing data from over 80 percent of U.S.-based fertility clinics; in 2013, these clinics performed a total of 92 percent of all IVF treatments.

The study’s results showed that 50 percent of IVF pregnancies utilizing fresh eggs resulted live births. By comparison, only 43 percent of IVF pregnancies using frozen (cryopreserved) oocytes resulted in live births.

According to lead author Dr. Vitaly A. Kushnir, M.D., it is not clear why the use of frozen eggs results in lower live birth rates.

“One possible explanation is…[that] oocyte quality may be negatively affected by cryopreservation and thawing,” says Dr. Kushnir.

Alternatively, experts hypothesize that the lower live birth rates can be traced directly to the reduced amount of eggs used for this IVF procedure.

“In traditional donor egg therapy (using fresh eggs), the recipient gets all the eggs the donor makes (typically 15-25),” said  James Toner, president of the Society for Assisted Reproductive Technology. “When frozen eggs are employed, only a batch of 6-8 eggs are provided. This has a big effect on outcomes!”

Given this uncertain state of affairs, Dr. Kushnir and his colleagues suggest that their research should be “viewed with caution,” noting that they did not account for potentially confounding factors, such as the age of egg donors and recipients and infertility diagnosis.