Women using frozen donated eggs for in vitro fertilization have fewer successful live births, compared with women receiving fresh eggs, say researchers analyzing the outcome of the two in vitro fertilization techniques.

The freezing of donor eggs is seen as a way to expand the time frame during which a woman might choose IVF to start a family, making it much more convenient for women and their doctors, as the egg cycles of the donor and recipient don't need to be synced.

However, the researchers found a higher pregnancy success rate when fresh eggs – rather than frozen – were used.

The success rate with fresh eggs was almost 50 percent, compared with 43 percent when using frozen eggs, the researchers reported in the Journal of the American Medical Association. 

They emphasized that the figures apply only to donated eggs: "This study looked only at egg donation IVF cycles, NOT at cycles in women who freeze/use their own eggs," said study lead author Dr. Vitaly A. Kushnir.

The study by the New York-based Center for Human Reproduction analyzed data from 380 U.S. fertility centers that performed more than 90 percent of all IVF procedures in the study year of 2013.

Of the 11,148 IVF cycles involving donated eggs in that year, 20 percent used eggs that had been frozen.

While fertility clinics have been routinely freezing embryos for some time, 2013 was the first year the American Society for Reproductive Medicine declared that freezing eggs was no longer considered an "experimental" procedure.

The difference in success rates should not, however, sway women considering either technique, according to Richard Paulson, the society's vice president.

The rates are "really quite similar, so they're not that far apart, and where they did find a little bit of a difference, it's hardly clinically significant," he explained.

Why there should be a difference at all is unclear, but the researchers suggest that the freezing and thawing techniques involved may have a negative effect on egg quality.

Some experts said the difference in success rates between frozen and fresh eggs is not significant and can be explained by other factors.

"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 Technologies. "When frozen eggs are employed, only a batch of six to eight eggs are provided. This has a big effect on outcomes!"