In 2012, the American Society for Reproductive Medicine announced that it was no longer “experimental” for a woman to freeze her eggs simply because she wanted to wait to have a child. Since then, demand for the procedure has skyrocketed, even though its costs remain high. Now, scientists say they have figured out—taking economic and biological considerations into account—the best age for women to freeze their eggs if they want to get pregnant as late in life as possible.

Two main factors determine when it’s best to freeze human eggs: how viable those eggs will be when thawed and how much it’s going to cost. The longer a woman waits to freeze her eggs, the less likely they are to result in a live birth. Yet, older women get more bang for their buck by freezing their eggs, because the procedure benefits them most; young women potentially waste money freezing their eggs because they’ll still likely be quite fertile a few years down the line.

To figure out the best age for women to freeze their eggs, researchers gathered data from national registries and surveys of pregnancies and fertility treatments, ongoing research studies of natural conception rates, and medical records from a national network of infertility practices. They then created decision models to compare the probability of live birth when a woman either froze her eggs for a specific number of years or decided to wait that specific number of years to try to get pregnant by natural methods or in vitro fertilization if needed. The team ran the models at ages ranging from 25 to 40 years old and assumed that conception attempts would begin 3, 5, or 7 years after the initial decision to freeze or not. (Those horizon years are arbitrary; no data exist on the average time between egg freezing and use.)

Egg freezing offered the greatest advantage over not freezing when the wait period was 7 years, the team reports in Fertility and Sterility. For that longer timespan, the researchers found that although egg freezing always costs more than not freezing, it also increased the chances of a live birth for all ages. For women under 32, however, that increase was minimal—less than 10%, the threshold the researchers considered “clinically significant”—and generally not worth the expense of freezing, because a woman’s natural fertility is still relatively high. The largest improvement came when eggs were frozen at age 37, increasing the probability of live birth at age 44 from 21.9% to 51.6%.

Fertility rates, however, are most strongly influenced by the age of a woman’s eggs. As a result, the nearly 52% probability of a successful pregnancy using those eggs frozen in time at age 37 is lower than what many women and their doctors consider acceptable. According to the study, most women would have to freeze their eggs by age 34 to have at least a 70% chance of live birth. Considering the researchers found that egg freezing provided the most improvement in live birth rates over not freezing after age 30, “we feel the sweet spot for those electing to freeze is age 31 to 33,” says co-author Tolga Mesen, an obstetrician-gynecologist at the Fertility Clinic at the University of North Carolina, Chapel Hill.

Oocyte cryopreservation is relatively new, and there is limited information about the use of frozen eggs for the prevention of age-related fertility decline. As a result, “studies like this are based on hypothetical assumptions,” says Dominic Stoop, director of the Centre for Reproductive Medicine at UZ Brussel in Belgium. Still, “the data inputs for these models are some of the best I’ve seen,” says Wendy Vitek, a reproductive endocrinologist at the University of Rochester Medical Center in New York. “Besides, women want these answers now. Modeling is an effective way to get the data that we’re interested in now.”