It’s no secret that women are waiting longer and longer to start families. In 1972, the average age of first-time mothers was 21. Today, it’s 26. In major cities like New York, it edges over 31, precisely when fertility begins to naturally decline. “As a woman gets older, it’s harder to get pregnant and stay pregnant,” says James A. Grifo, MD, PhD, director of the NYU Langone Fertility Center. “Older eggs have riskier outcomes.”

Grappling with the hard reality of biology, more and more women are seeking oocyte cryopreservation, or egg freezing, as a way to preserve their fertility in the hopes of having children in the future. Once considered experimental, the science of freezing eggs is now standard practice for many fertility clinics, a shift based in large part on the groundbreaking work of embryologists and doctors at the Fertility Center.

In 2005, led by Dr. Grifo, they were among the first fertility specialists to show that frozen eggs could be thawed, fertilized, and result in a successful birth. Since then, the use of egg cryopreservation has boomed. The number of egg-freezing cycles completed in the United States more than doubled from 2005 to 2020, from roughly 7,600 to nearly 17,000.

Yet despite the technology’s growing popularity, questions linger over the viability of frozen eggs, partly because most women never take their eggs out of storage. Seeking to bring clarity to the field, researchers at the Fertility Center analyzed data from 543 of their patients who conceived through in vitro fertilization (IVF) using their own supply of frozen eggs. All told, the physicians evaluated 800 egg-freezing cycles, 605 egg thaws, and 436 embryo transfers between 2005 and 2020. The study—the largest ever conducted by a single center—focused exclusively on women who chose cryopreservation to stave off age-related declines in fertility.

Their landmark results, published in Fertility and Sterility, showed that 39 percent of patients had a successful birth using their own frozen eggs. These results match the success rates for women of comparable ages who have IVF using “fresh” eggs. The success rate jumped to 50 percent for women younger than 38 and 70 percent for those under 38 who thawed more than 20 eggs.

The results offer some assurances for women in their late 30s and early 40s. The median age of the patients at the time when they froze their eggs was 38, slightly older than the average of 35. The researchers reported 14 births for women who froze eggs between the ages of 41 and 43. “Successful births at this age are still possible, but freezing eggs sooner can dramatically improve the odds,” says co-author Jennifer K. Blakemore, MD, a reproductive endocrinologist at the Fertility Center.

The researchers caution against waiting too long to hit pause on the fertility clock. “If you do IVF at 40, you have a much better chance of having a baby if you use eggs that you froze in your early 30s,” notes Dr. Grifo. “It’s the age at which you freeze your eggs that predicts your success rate, not the age at which you use them.”