The Strange World Of 'Snowflakes': Human Babies In Cryogenic Storage

Lisa Holligan already had two children when she decided to try for another baby. Her first two pregnancies had come easily. But for some unknown reason, the third didn’t. Holligan and her husband experienced miscarriage after miscarriage after miscarriage.

Like many other people struggling to conceive, Holligan turned to in vitro fertilization, or IVF. The technology allows embryologists to take sperm and eggs and fuse them outside the body, creating embryos that can then be transferred into a person’s uterus. The fertility clinic treating Holligan was able to create six embryos using her eggs and her husband’s sperm. Genetic tests revealed that only three of these were “genetically normal.” After the first was transferred, Holligan got pregnant. Then she experienced yet another miscarriage. “I felt numb,” she recalls. But the second transfer, which took place several months later, stuck. And little Quinn, who turns four in February, was the eventual happy result. “She is the light in our lives,” says Holligan.

Holligan, who lives in the UK, opted to donate her “genetically abnormal” embryos for scientific research. But she still has one healthy embryo frozen in storage. And she doesn’t know what to do with it. Should she and her husband donate it to another family? Destroy it? “It’s almost four years down the line, and we still haven’t done anything with [the embryo],” she says. The clinic hasn’t been helpful—Holligan doesn’t remember talking about what to do with leftover embryos at the time, and no one there has been in touch with her for years, she says.

Holligan’s embryo is far from the only one in this peculiar limbo. Millions—or potentially tens of millions—of embryos created through IVF sit frozen in time, stored in cryopreservation tanks around the world. The number is only growing thanks to advances in technology, the rising popularity of IVF, and improvements in its success rates. At a basic level, an embryo is simply a tiny ball of a hundred or so cells. But unlike other types of body tissue, it holds the potential for life. Many argue that this endows embryos with a special moral status, one that requires special protections. The problem is that no one can really agree on what that status is. To some, they’re human cells and nothing else. To others, they’re morally equivalent to children. Many feel they exist somewhere between those two extremes.

There are debates, too, over how we should classify embryos in law. Are they property? Do they have a legal status? These questions are important: There have been multiple legal disputes over who gets to use embryos, who is responsible if they are damaged, and who gets the final say over their fate. And the answers will depend not only on scientific factors, but also on ethical, cultural, and religious ones. The options currently available to people with leftover IVF embryos mirror this confusion. As a UK resident, Holligan can choose to discard her embryos, make them available to other prospective parents, or donate them for research. People in the US can also opt for “adoption,” “placing” their embryos with families they get to choose. In Germany, people are not typically allowed to freeze embryos at all. And in Italy, embryos that are not used by the intended parents cannot be discarded or donated. They must remain frozen, ostensibly forever. 

While these embryos persist in suspended animation, patients, clinicians, embryologists, and legislators must grapple with the essential question of what we should do with them. What do these embryos mean to us? Who should be responsible for them? Meanwhile, many of these same people are trying to find ways to bring down the total number of embryos in storage. Maintenance costs are high. Some clinics are running out of space. And with a greater number of embryos in storage, there are more opportunities for human error. They are grappling with how to get a handle on the growing number of embryos stuck in storage with nowhere to go.

The embryo boom

There are a few reasons why this has become such a conundrum. And they largely come down to an increasing demand for IVF and improvements in the way it is practiced. “It’s a problem of our own creation,” says Pietro Bortoletto, a reproductive endocrinologist at Boston IVF in Massachusetts. IVF has only become as successful as it is today by “generating lots of excess eggs and embryos along the way,” he says. 

To have the best chance of creating healthy embryos that will attach to the uterus and grow in a successful pregnancy, clinics will try to collect multiple eggs. People who undergo IVF will typically take a course of hormone injections to stimulate their ovaries. Instead of releasing a single egg that month, they can expect to produce somewhere between seven and 20 eggs. These eggs can be collected via a needle that passes through the vagina and into the ovaries. The eggs are then taken to a lab, where they are introduced to sperm. Around 70% to 80% of IVF eggs are successfully fertilized to create embryos.

The embryos are then grown in the lab. After around five to seven days an embryo reaches a stage of development at which it is called a blastocyst, and it is ready to be transferred to a uterus. Not all IVF embryos reach this stage, however—only around 30% to 50% of them make it to day five. This process might leave a person with no viable embryos. It could also result in more than 10, only one of which is typically transferred in each pregnancy attempt. In a typical IVF cycle, one embryo might be transferred to the person’s uterus “fresh,” while any others that were created are frozen and stored.

IVF success rates have increased over time, in large part thanks to improvements in this storage technology. A little over a decade ago, embryologists tended to use a “slow freeze” technique, says Bortoletto, and many embryos didn’t survive the process. Embryos are now vitrified instead, using liquid nitrogen to rapidly cool them from room temperature to -196 °C in less than two seconds. Vitrification essentially turns all the water in the embryos into a glasslike state, avoiding the formation of damaging ice crystals. Now, clinics increasingly take a “freeze all” approach, in which they cryopreserve all the viable embryos and don’t start transferring them until later. In some cases, this is so that the clinic has a chance to perform genetic tests on the embryo they plan to transfer.

Once a lab-grown embryo is around seven days old, embryologists can remove a few cells for preimplantation genetic testing (PGT), which screens for genetic factors that might make healthy development less likely or predispose any resulting children to genetic diseases. PGT is increasingly popular in the US—in 2014, it was used in 13% of IVF cycles, but by 2016, that figure had increased to 27%. Embryos that undergo PGT have to be frozen while the tests are run, which typically takes a week or two, says Bortoletto: “You can’t continue to grow them until you get those results back.”

And there doesn’t seem to be a limit to how long an embryo can stay in storage. In 2022, a couple in Oregon had twins who developed from embryos that had been frozen for 30 years.

Put this all together, and it’s easy to see how the number of embryos in storage is rocketing. We’re making and storing more embryos than ever before. When you combine that with the growing demand for IVF, which is increasing in use by the year, perhaps it’s not surprising that the number of embryos sitting in storage tanks is estimated to be in the millions. I say estimated, because no one really knows how many there are. In 2003, the results of a survey of fertility clinics in the US suggested that there were around 400,000 in storage. Ten years later, in 2013, another pair of researchers estimated that, in total, around 1.4 million embryos had been cryopreserved in the US. But Alana Cattapan, now a political scientist at the University of Waterloo in Ontario, Canada, and her colleagues found flaws in the study and wrote in 2015 that the number could be closer to 4 million.  

That was a decade ago. When I asked embryologists what they thought the number might be in the US today, I got responses between 1 million and 10 million. Bortoletto puts it somewhere around 5 million. Globally, the figure is much higher. There could be tens of millions of embryos, invisible to the naked eye, kept in a form of suspended animation. Some for months, years, or decades. Others indefinitely.

In theory, people who have embryos left over from IVF have a few options for what to do with them. They could donate the embryos for someone else to use. Often this can be done anonymously (although genetic tests might later reveal the biological parents of any children that result). They could also donate the embryos for research purposes. Or they could choose to discard them. One way to do this is to expose the embryos to air, causing the cells to die.

Studies suggest that around 40% of people with cryopreserved embryos struggle to make this decision, and that many put it off for five years or more. For some people, none of the options are appealing.
In practice, too, the available options vary greatly depending on where you are. And many of them lead to limbo.

Take Spain, for example, which is a European fertility hub, partly because IVF there is a lot cheaper than in other Western European countries, says Giuliana Baccino, managing director of New Life Bank, a storage facility for eggs and sperm in Buenos Aires, Argentina, and vice chair of the European Fertility Society. Operating costs are low, and there’s healthy competition—there are around 330 IVF clinics operating in Spain. (For comparison, there are around 500 IVF clinics in the US, which has a population almost seven times greater.)

Baccino, who is based in Madrid, says she often hears of foreign patients in their late 40s who create eight or nine embryos for IVF in Spain but end up using only one or two of them. They go back to their home countries to have their babies, and the embryos stay in Spain, she says. These individuals often don’t come back for their remaining embryos, either because they have completed their families or because they age out of IVF eligibility (Spanish clinics tend not to offer the treatment to people over 50). 

In 2023, the Spanish Fertility Society estimated that there were 668,082 embryos in storage in Spain, and that around 60,000 of them were “in a situation of abandonment.” In these cases the clinics might not be able to reach the intended parents, or might not have a clear directive from them, and might not want to destroy any embryos in case the patients ask for them later. But Spanish clinics are wary of discarding embryos even when they have permission to do so, says Baccino. “We always try to avoid trouble,” she says. “And we end up with embryos in this black hole.”

This happens to embryos in the US, too. Clinics can lose touch with their patients, who may move away or forget about their remaining embryos once they have completed their families. Other people may put off making decisions about those embryos and stop communicating with the clinic. In cases like these, clinics tend to hold onto the embryos, covering the storage fees themselves.

Nowadays clinics ask their patients to sign contracts that cover long-term storage of embryos—and the conditions of their disposal. But even with those in hand, it can be easier for clinics to leave the embryos in place indefinitely. “Clinics are wary of disposing of them without explicit consent, because of potential liability,” says Cattapan, who has researched the issue. “People put so much time, energy, money into creating these embryos. What if they come back?” Bortoletto’s clinic has been in business for 35 years, and the handful of sites it operates in the US have a total of over 47,000 embryos in storage, he says. “Our oldest embryo in storage was frozen in 1989,” he adds. 

Some people may not even know where their embryos are. Sam Everingham, who founded and directs Growing Families, an organization offering advice on surrogacy and cross-border donations, traveled with his partner from their home in Melbourne, Australia, to India to find an egg donor and surrogate back in 2009. “It was a Wild West back then,” he recalls. Everingham and his partner used donor eggs to create eight embryos with their sperm.

Everingham found the experience of trying to bring those embryos to birth traumatic. Baby Zac was stillborn. Baby Ben died at seven weeks. “We picked ourselves up and went again,” he recalls. Two embryo transfers were successful, and the pair have two daughters today.

But the fate of the rest of their embryos is unclear. India’s government decided to ban commercial surrogacy for foreigners in 2015, and Everingham lost track of where they are. He says he’s okay commercial surrogacy for foreigners in 2015, and Everingham lost track of where they are. He says he’s okay with that. As far as he’s concerned, those embryos are just cells.

Jessica Hamzelou writes for MIT Technology Review.