Women have been led astray on egg-freezing, bombshell study finds. There are 2 major things they have to know

The U.S., like many developed countries, is facing a demographic crisis.

Population growth is stalling, and more Americans than ever are saying they don’t want kids.

Many American women who do want kids are delaying it, and opting to freeze their eggs—a seemingly magical solution that means they can make a decision about parenthood on their own terms.

But this strategy may not be as failsafe as expected. Groundbreaking research, the first of its kind in the U.S. finds that for the majority of women, egg-freezing sadly won’t result in a child being born.

Only around one in three of those who choose to freeze their eggs will wind up having a child after going through the entire process, the study finds—and that success rate drops drastically for older women.

In a recent research paper, a team of experts from NYU’s Grossman School of Medicine and Langone Fertility Center said the proportion of patients who ultimately had a baby after going through the whole process of freezing and using their eggs was just 39%—and the success rate was heavily influenced by the patient’s age.

How could this be? It has to do with how early (or late) women choose to freeze eggs, and how many they freeze when they do.

The odds go down as the age increases

Between 2010 and 2016, the number of American women freezing their eggs skyrocketed 880%, driven by the American Society for Reproductive Medicine removing the “experimental” label from egg freezing procedures in 2012. According to the American Society for Reproductive Technology, 10,936 U.S. women froze their eggs in 2017 alone.

Even employers have taken note of the growing interest in the fertility preservation trend, with more and more companies offering fertility benefits like egg freezing to employees as part of their benefits packages.

But freezing eggs is a long and costly procedure—it can come with a price tag in the tens of thousands of dollars—and with the market expected to keep booming, some experts have warned that clinics are not transparent enough about success rates and the potential risks involved. That means older women are more likely to be able to afford to freeze eggs, but the NYU scientists found that gets to the crux of the problem with the process.

The NYU scientists analyzed data from 543 patients who thawed a collective total of 605 eggs at their clinic between 2005 and 2020. The median age of the women in the sample when they froze their eggs was 38, and the median time between freezing eggs and returning to the clinic to use them was 4.2 years. The live birth rate per patient was just 39%. Only patients who either had a live birth or used up all of their inventory at the Langone facility were included in the birth rate calculation.

Those who froze their eggs when they were under the age of 38 were more likely to have a baby as a result of the process, with those in the younger cohort seeing a birth rate of more than 50%.

However, just 8% of patients were under the age of 35 when they underwent their first egg freezing cycle; the youngest patient was 27 years old. Eight in 10 patients were between 35 and 40 years old, and 12% were over the age of 41.

The study adds to previous research that found women who freeze their eggs over the age of 40 are unlikely to have a successful live birth. Scientists from Imperial College London said in August that while egg freezing was a viable option for many, women "must be counselled about the poor outcomes when they embark on egg freezing above the age of 40."

Not freezing enough

Beyond the age at which a woman chooses to freeze eggs, the number of eggs women froze was also central to how successful they were. The study found that the more eggs were frozen, the higher the birth rate was.

However, many women did not have enough eggs stored to boost their chances, and the number of eggs being thawed was much lower among women who first froze their eggs at an older age.

Women of all ages who had at least 20 mature eggs to thaw saw their chances of giving birth increase to more than 58%, compared to a 24% birth rate among those who froze fewer than 10.

The median number of mature eggs thawed per patient was 12. Women who were younger than 38 had a median 14 mature eggs to thaw, while those age 41 or older thawed a median of 9 mature eggs.

Women under the age of 38 who thawed more than 20 eggs had a birth rate of 70%, compared to a birth rate of 36% for those in the same age group who used less than 10 eggs.

Again, older women saw their success rates massively depleted compared to their younger counterparts, with women over 41 who froze fewer than 10 eggs going on to have a live birth rate of just 13%. In contrast, one in three women over 41 who froze at least 20 eggs had successful pregnancies.

Around one in five of all the eggs that were thawed did not survive, and just a quarter of patients had all of their eggs survive the thawing process, while 1% had no egg survival at all. Of the eggs that survived the thawing, 65% were successfully fertilized.

Researchers who authored the paper acknowledged that further studies with larger patient samples were needed, but they said their study was the largest of its kind to have been carried out in the U.S. to date.

“As [egg freezing] utilization increases, outcome data should be published so patients can make informed decisions about [its] value in securing their reproductive futures,” they said, before adding that their findings showed egg freezing was a “viable fertility preservation method.”

The findings from the study show egg freezing has similar, age-dependent, success rates to IVF—the process of extracting and fertilizing eggs before implanting the resulting embryos into a patient’s womb.

Data from the Society for Assisted Reproductive Technology shows that IVF has a success rate of almost 70% for women under the age of 35, while women older than 43 have a birth rate of less than 10%.

According to the U.S. Centers for Disease Control and Prevention, around 2% of infants born every year in the United States are conceived using fertility treatments like IVF and egg freezing.

This story was originally featured on Fortune.com