Eggs are prepared for thawing at Shady Grove Fertility Center. (Andre Chung for The Washington Post)

Facebook and Apple triggered a discussion about women's careers and fertility last year when they announced they would cover egg freezing for female employees. Sometimes  the most pivotal part of a woman’s career can inconveniently coincide with the window during which it's biologically easiest and safest to have kids, and egg freezing is considered one potential option for extending that time frame.

A study published Tuesday in the Journal of the American Medical Association sounds a cautionary note about the process, however, finding that pregnancies from egg freezing were slightly less successful than traditional IVF with fresh eggs. There was a nearly 50 percent pregnancy success rate per started IVF cycle with fresh eggs, compared with a 43 percent success rate with frozen eggs.

To track the success of both techniques, researchers for the Center for Human Reproduction in New York mined the data at 380 U.S. fertility centers that in 2013 performed 92 percent of all IVF procedures. That’s the year egg-freezing was deemed by the American Society for Reproductive Medicine to be nonexperimental.

[How fear fuels the business of egg freezing]

Richard Paulson, vice president of the American Society for Reproductive Medicine, said that despite the difference in pregnancy rates, he found the study reassuring. The success rate was "really quite similar, so they’re not that far apart, and where they did find a little bit of a different, it’s hardly clinically significant," Paulson said.

He said the biggest surprise to him was that about one-fifth of the IVF cycles were from frozen eggs -- he would have expected the number to be much lower, and it shows how widely the technology is being used.

Still, he and others noted, the study had real limitations -- it wasn't a randomized trial in which researchers were rigorously comparing the success of fresh and frozen eggs in matched groups of women. But Paulson said the real-world data was important because it suggests that the technology is working in a broad array of places and not just a single research clinic. But Paulson noted that only in five to 10 years will we really be able to understand thoroughly the data on the success rates of women who are freezing their eggs now for later use.

"To me, it's reassuring. But these are young, fertile donors. Can you extrapolate from a 28-year-old woman to a 40-year-old woman?" Paulson said.

The researchers put forward a few possible reasons that the frozen eggs would lead to fewer pregnancies: The thawing process might negatively affect the eggs, or women may be using lower-quality eggs because there are fewer to start with.

James Toner, president of the Society for Assisted Reproductive Technologies, said that he believes the difference in pregnancy rates is a "red herring."

"In traditional donor egg therapy (using fresh eggs), the recipient gets all the eggs the donor makes (typically 15-25)," Toner wrote in an e-mail. "When frozen eggs are employed, only a batch of 6-8 eggs are provided.  This has a big effect on outcomes!"

Although many fertility doctors found the results reassuring, they did caution that although egg freezing may be a potential insurance policy against the effects of age on fertility, it is not a sure thing.

"We need to recognize that cryopreservation is a procedure that requires specific technical expertise and proficiency," said Rebeca Sokol, president of the American Society for Reproductive Medicine. "While great improvements in technique and outcomes have been made in recent years, and we expect to see more in the future, patients should be cautioned that putting their eggs into storage today is no guarantee of having a baby tomorrow."

Read more:

Biological clock ticking? What you need to know about freezing your eggs.

Controversial gene therapy might help women have healthy children

Why China stops single women from freezing their eggs

Want to freeze your eggs? Only a few firms, such as Facebook and Apple, help pay for it.