By Charlotte Alter
July 16, 2015

You could think of a human egg as a tiny bubble of hope. Just a single cell full of liquid, less than half the size of this comma, it contains the DNA of your parents and their parents, your children and their children. It’s a world-in-waiting, a fragile bud of human potential. And, as with all not-quite-realized things, its chief enemy is time.

Egg quality and quantity deteriorate with age, and they are vulnerable to attacks on the body. That’s why, for more than 20 years, doctors have frozen eggs for cancer patients who want to have kids after chemotherapy and others with medical conditions that could impair fertility. But since 2012, when the American Society for Reproductive Medicine (ASRM) lifted the “experimental” label on egg freezing after a new quick-freeze method was developed, elective egg freezing has taken off.

The process has been hailed as a game changer for millennial women–a generation who grew up watching their mothers fret about “having it all” but who feel secure in the assumption that any problem can be hacked with the right technology. For them, egg freezing promises to solve the problem of the biological clock and complete the work of the sexual revolution. While contraception allowed women to avoid becoming young mothers, egg freezing could allow them to become older ones. It’s a hopeful message for women looking over the fertility cliff, one that egg-freezing evangelists are glad to deliver. Freezing, women are told, is a fountain of youth.

Like most such promises, it doesn’t quite deliver. Some women will take home babies from their frozen eggs, but many won’t. And for a procedure pitched to anxious women as an “insurance policy,” there is shockingly little data on exactly what their chances are.

But with the recent corporate embrace of egg freezing for employees–Apple and Facebook announced last year that they’ll be covering the procedure–this new subset of the already robust fertility industry is growing fast, even if the data haven’t quite caught up with the excitement. Gatherings hosted by EggBanxx, a New York City–based fertility startup that acts as a broker for egg-freezing services, feature wine, hors d’oeuvres and a panel of doctors discussing “special discounts” and “financing options” for patients who buy procedures through their company. Egg freezing can cost $10,000 to $15,000 and is not covered by most health care plans.

So far, EggBanxx is the only fertility marketplace to include an explicit focus on egg freezing. In March, its parent company, Fertility Authority, combined with another company to form Progyny, an online network through which patients can pick fertility doctors, clinics and procedures. Progyny is privately held and is funded in part by Merck Serono Ventures. (MS Ventures is the strategic corporate-venture arm of the biopharmaceutical division of Merck KGaA, which makes three major fertility drugs.)

For every woman who freezes her eggs, EggBanxx gets a cut. Founder and CEO Gina Bartasi, who is now running Progyny, calls it “Uber for the fertility industry” because it aims to seamlessly connect patients with medical providers. She estimates that by 2018, 76,000 U.S. women will freeze their eggs every year. “Freezing your eggs is the biggest breakthrough in women’s health since the birth control pill,” she says.

But the Pill has been proved to be more than 90% effective over 50 years–egg freezing, not so much. Even though it’s been done experimentally for years, the elective procedure is so new that there are no published national live-birth rates, although doctors say they’re similar to those for in vitro fertilization–around 40% for women under 35, dropping to less than 5% after 42. But the data is still murky: there are simply not enough births yet to indicate egg freezing is anything close to a sure thing.

Nonetheless, for some young women, the potential benefits are irresistible. Vicki Rokhlin, a 28-year-old who works in advertising tech in New York City, got 19 eggs retrieved this year, not through EggBanxx. “I just don’t want to suffer any consequences for being ambitious in my 20s,” she says. She said the decision made her feel both accomplished and secure, similar to how her friends felt when they got engaged or married. “I feel that same relief,” she says. “Right after the surgery was over, I just thought, Wow, I’ve got an entire chunk of my life figured out. And I did it. No man did it, just me.”

The psychological effects of egg freezing are striking. Almost all the women interviewed for this article agreed that preserving their eggs gave them a sense of control, time to focus on their careers, a release from the constant ticktock in the back of their minds. The only thing it hasn’t given them–and isn’t guaranteed to–is a baby.

Women are getting married and having children later than ever, but our bodies are just the same as they’ve always been: we are born with all the eggs we’ll ever have (approximately 1 million to 2 million) and we lose them steadily as we age. Quantity aside, egg quality also diminishes with time, which means there’s an increased risk of chromosomal abnormalities in older eggs that can lead to miscarriages or birth defects. While studies vary on the rate of decline, all major medical organizations agree that it gets significantly harder for women to get pregnant and carry to term as they approach 40.

Yet later motherhood is becoming the new norm. One in seven American children is born to a mother over 35, a 64% increase since 1990. From 2000 to 2012, first births to mothers 40 to 44 rose 35%, according to the Centers for Disease Control. And among the highly educated, affluent women who can afford to freeze their eggs, there is a growing stigma around settling down too early, especially if they’re building a career. That leaves women in a tight squeeze: having children too early can seem irresponsible, but having them too late may be impossible. It’s a prisoner’s dilemma, and egg freezing can seem like a get-out-of-jail-free card.

But what seems like an easy fix is not necessarily a simple procedure. Freezing your eggs is basically like pressing pause in the middle of an IVF cycle, allowing the patient to serve as her own future egg donor. After using fertility drugs to stimulate the follicles, doctors extract the eggs and flash-freeze them to store for later use. The patient returns to get her eggs thawed when she wants to get pregnant, and from that point on the procedure is just like IVF: the eggs are fertilized with sperm to create embryos and then transferred to the uterus in the hope of starting a pregnancy.

Doctors say that freezing your eggs preserves the IVF success rate at the age at which you froze them. Frozen eggs seem to behave like fresh ones, and initial studies show no increased risk of birth defects. “It’s comparable to IVF, and it’s safe,” says Dr. Jamie Grifo of the New York University Fertility Center. “And the age at which you freeze your eggs is what determines the chromosomal-abnormality rate.”

But fertility also depends on the age of the mother’s body. It could still be difficult for a woman in her 40s to get pregnant and carry a child, even using an egg she froze in her 30s, because older mothers have a higher risk of pregnancy-related health problems. And although the comparison to IVF sounds promising, IVF itself is hardly a foolproof procedure–even in younger women, most embryo transfers don’t result in a live birth.

The Society for Assisted Reproductive Technology (SART), the primary medical organization of assisted reproduction, is only just starting to track egg freezes, thaws and live births, but the vast majority of patients have not yet returned to thaw their eggs. To date, there has been no widespread, definitive research published on the number of women who take home babies from their own frozen eggs. Instead, success rates are cobbled together from data on IVF procedures (usually done with a woman’s own fresh eggs) or egg donations (often from women in their early 20s). This allows for inconsistent standards in reporting births, and doctors aren’t always fully transparent with patients about the lack of data.

Initial numbers compiled exclusively for TIME by Dr. Kevin Doody, chairman of the SART Registry from 2012 to 2014, offer the most concrete picture so far, since more than 90% of fertility clinics report their data to SART. According to Doody, egg-freezing cycles have multiplied tenfold in the past five years, from just over 500 in 2009 to just under 5,000 in 2013. Thaws have nearly quadrupled, from 123 in 2009 to 414 in 2013.

But of the women who thaw their eggs, only a small portion end up carrying a child to term. Of the 353 egg-thaw cycles in 2012, only 83 resulted in a live birth. After 414 thaws in 2013, 99 babies were born. Those are the most comprehensive live-birth rates for egg freezing, and they’re just under 24%. It should be noted that some of these eggs may have been frozen with an older slow-freeze method, which has a much lower success rate. Still the odds are not great.

But some patients are still under the misconception that 20 frozen eggs equals 20 future babies. Ivy, a 39-year-old writer in Toronto, says she was already well into her egg-freezing process before her doctors told her that even if they retrieved 20 eggs, she might only get one or two live births. “It felt like that information, which was very critical, had not been fully revealed to me until that point,” she said. “The calculus hadn’t been done in advance, and perhaps if they do that, they’re not going to reel in as many customers.” (Ivy asked that her full name not be used, for fear of jeopardizing her relationship with the clinic storing her frozen eggs.)

“A lot of these clinics massage their data,” says Dr. Mary Jane Minkin, an ob-gyn who teaches obstetrics at Yale School of Medicine. “All of this stuff is a gamble, and I would certainly hate like heck to have anybody count on this as an absolute guarantee.”

But many women do think of egg freezing as a guarantee, partly because of the “insurance policy” marketing rhetoric. It helps that women want to believe that it will work. The appeal is as much about feminism as it is about fertility: for a generation of women who feel they can control everything but their biology, freezing their eggs seems like a way to shatter the last gender barrier.

That wishful thinking is reinforced by corporate endorsement of egg freezing for employees. Other companies may soon follow Apple and Facebook’s lead. Progyny is entering the corporate health-benefits space, and Bartasi says over the past three months she’s fielded over a dozen requests from other tech companies that want to cover egg freezing to help recruit talented women.

And the message is spreading. Bay Area fertility specialist Dr. Aimee Eyvazzadeh started hosting egg-freezing parties in November in order to “raise fertility awareness”–now she gets requests to host soirées everywhere from Washington, D.C., to Denmark. The parties feature wine, snacks, goody bags and a PowerPoint presentation by “Dr. Aimee,” a bubbly 39-year-old reproductive endocrinologist who wears bright lipstick and an “Egg-Whisperer” shirt. Everyone leaves with a T-shirt, a tote bag and a tiny bar of soap in the shape of an egg, and Eyvazzadeh says she pays for everything out of pocket.

“It’s Fertility 101, in a Ladies Night format,” Eyvazzadeh says. As part of her awareness platform, she encourages women to get regular FSH tests, which measure the level of the follicle-stimulating hormone and can help women learn how fast their ovaries are aging. “If I can talk to those women today who might be in my office in 10 years, that’s why I’m spending this money,” she says, adding that she tries to dissuade patients from thinking of egg freezing as a sure thing. “It’s a chance. That’s all it is.”

The chance is enough for some women. “For me it all came down to knowing that if and when I reached the point where I was ready to have children, I had done everything that I could,” says MeiMei Fox, a 42-year-old writer in Los Angeles who froze her eggs at 37 after a divorce. “If it didn’t work out, so be it.”

Some women see it as just another chore of modern womanhood. “There’s almost a ‘check the box’ attitude about it: I got my teeth cleaned, I filed my taxes, I froze my eggs,” says Sarah Elizabeth Richards, author of Motherhood, Rescheduled, a book about four egg-freezing experiences, including her own. “Protecting your fertility is just one more aspect of being a successful woman. Almost like taking out an IRA.”

Most of the women at egg-freezing parties are in their mid-to-late 30s and already worried about their fertility, but EggBanxx wants to get women to freeze when they’re younger, ideally in their late 20s. The company has adopted a deliberate marketing strategy to reach young women through social media, and it recently launched a mobile platform so that paying for egg freezing is as easy as buying a Groupon. The average age of EggBanxx attendees has dropped from 38 to 33 in the past year.

And yet when doctors at EggBanxx events are asked about live-birth rates, most hedge their answers by quoting IVF success rates or egg-donation data. When asked how many babies have been born from eggs they’ve frozen, few quote a number above two or three. “When doctors stand up in the room and say, ‘Well, well, well,’ it’s because they don’t have the data,” EggBanxx founder Bartasi acknowledges.

Still, the consensus is that if you’re going to freeze, younger is better, which is why Rokhlin put her eggs on ice while still in her 20s. “I’m doing this now so that I’m not 34 wanting to have kids and putting so much pressure on my body,” she says. “I really believe this will start being a gift which parents give to their daughters when they graduate college, so that they don’t feel this cataclysmic pressure to follow a timeline.”

That’s what EggBanxx is hoping. “In the next couple of generations, if you’re 30 and professional, you’ll just do this as an insurance policy, just like you have life insurance and health insurance and homeowner’s insurance,” Bartasi says.

But even doctors who recommend the procedure–and profit from it–reject that description. “As an insurance policy goes, it’s not a great one,” says Grifo of NYU. “When your house burns down, you get a new house, you don’t get 40% of a new house.” Grifo is one of the leaders in the field: in a decade of freezing, about 85 babies have been born from eggs frozen at Grifo’s clinic, and more than 30 pregnancies are ongoing.

If doctors are trying to manage expectations, some women who have tried multiple rounds of IVF are downright skeptical. “There is unfortunately a real growth industry here around those who see dollars and not necessarily babies,” says Pamela Tsigdinos, an infertility blogger and the author of Silent Sorority, who attempted five IVF cycles with fresh eggs. “You don’t see women on stage like me talking about the absolute heartbreak,” she says. “The doctors keep telling you your eggs and embryos look fabulous, and then you’re left sitting in the dark room with the phone and someone has just told you you’re not pregnant.”

As with any medical procedure, there are risks associated with egg freezing. Rokhlin got ovarian torsions after her egg retrieval, a condition in which her ovaries twisted and swelled to the size of grapefruits. She had to go to the ER and get her ovaries drained with what she calls a “shish-kebab-size needle.” That happens to fewer than 1% of egg-retrieval patients, but there is a danger of ovarian hyperstimulation and other side effects.

After they’re retrieved, the eggs are frozen using a relatively new process known as vitrification, which flash-freezes the egg to keep it intact. Then they’re stored in liquid nitrogen for as long as the patient wants, or for as long as she can afford to pay the storage fees, which can be up to $1,000 a year. But there are risks to leaving your frozen eggs in a storage facility as well, and if you move away, shipping your eggs can be disastrous. Fox attempted to thaw her eggs 2½ years after she froze them, but they were accidentally destroyed in transit (which is relatively rare). She said it was one of the worst days of her life.

Although anecdotal evidence suggests eggs can be frozen indefinitely without any adverse effects, there are not yet any large studies proving how long they can stay frozen. And eggs can’t be tested for genetic abnormalities until they’re in the blastocyst phase–a fertilized but pre-embryonic stage–which is another hitch in the plan for egg freezers. While genetic testing of unfertilized eggs is technically possible, it’s far too expensive to become part of the regular procedure in the U.S. But to freeze eggs without testing them is a big gamble, since even young women have a large proportion of chromosomally flawed eggs, and that number just increases with age. That means there’s a good chance women could be freezing–and pinning their hopes on–bad eggs.

Rokhlin says the promise of genetic testing of embryos is what sold her on egg freezing in the first place. “I went into egg freezing thinking it would only buy me time. But actually it’s the guarantee of a healthy child that made me pull the trigger,” she says. “For me, it’s not an insurance policy. It’s Plan A.”

There is no such certainty, say many doctors. “People are being set up to believe that if you freeze an egg, it’s going to give you an assurance of having a baby down the line,” says Dr. Geoffrey Sher, who runs Sher Fertility. “But if you don’t know that egg is genetically normal, you may be giving people a false promise.”

At every step of the process, a few eggs are lost. Of all the eggs extracted, only some are healthy enough to be frozen. (Five of Rokhlin’s eggs were not deemed viable.) Of all the eggs that are frozen, only 75% to 80% make it through the thaw. Of all the eggs that are thawed, only a portion make it to the blastocyst phase, and only a portion of those are deemed genetically healthy enough to transfer to the womb. And of the healthy embryos transferred, only some will survive to a live birth, especially if the mother is older. The ASRM estimates that even in women under 38, each egg has just a 2% to 12% chance of resulting in a live birth.

For Rokhlin, her frozen eggs feel like a brood just waiting to be born. “A girlfriend of mine sent me a ‘Happy Mother’s Day’ note,” she says. “She said, ‘Congratulations to the most heroic mother of all for sacrificing so much for your 19 little babies.'”

All this might explain why, when it removed the “experimental” label from egg freezing in 2012, the ASRM also warned against elective egg freezing: “Marketing this technology for the purpose of deferring childbearing may give women false hope.”

For many women, freezing their eggs is as much about fear as it is about hope. Fear that they won’t find the right partner, fear that prioritizing pregnancy could have consequences at work, fear that their fertility will suddenly plummet. Even for women who know there’s only a small chance their frozen egg will become a baby, that chance is better than nothing. For them, freezing their eggs is as much a way to find peace of mind in the short term as it is about a future family.

“I do think it’s easy to buy into the dream of ‘Oh my god, all I have to do is sign here and give you $15,000,” says Danielle, an EggBanxx attendee who asked that her full name not be used because she doesn’t want her fertility decision to impact her dating life. “If you talk to a bunch of women in their 30s, all they really want is to not be having this problem. It’s an easy sell.”

That feeling of security can be beneficial even if a baby never materializes. “Even though it didn’t work for all women, I think most women live their lives assuming it will work, so they live those years without so much pressure,” says Richards, who hasn’t tried to thaw her eggs in the two years since she published Motherhood, Rescheduled. “They still got the psychological benefit of assuming it would work.”

But that very benefit, the idea that childbearing can be postponed without risk, can also have devastating outcomes. If a woman waits until her 40s to thaw her eggs, the frozen eggs could be her last shot to have her own biological child. And unlike IVF cycles in younger women, a finite number of frozen eggs means a finite number of chances at fertilization and implantation: if it doesn’t work, that’s it.

That’s why Minkin thinks the much discussed peace of mind amounts to a false sense of security. “If they’re going to rely on this as something that’s definitive, then they’re going to say, ‘Okay, I don’t have to worry about this,'” she says. “And then find out 15 years from now that those eggs are totally unusable.”

At a recent EggBanxx event in New York, a former patient told the crowd freezing her eggs helped her feel empowered, secure and in control. Danielle was not impressed. “I thought she was going to be like, ‘It worked for me,'” she says. “I was waiting for that part of the story.”

Write to Charlotte Alter at charlotte.alter@time.com.

This appears in the July 27, 2015 issue of TIME.

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