The truth about egg freezing

Whatever you do, do not listen to the articles that say that egg and embryo freezing do not work.

This is the fourth article we have released from Issue 23, which print subscribers started receiving last week. Not yet a subscriber? You can sign up for the magazine here.

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Egg freezing is much more effective than most people think. Articles in major publications like The New York Times (‘Sobering study shows challenges of egg freezing’) and Vox (‘The failed promise of egg freezing’) have reported that only about two fifths of women will be able to successfully have children from their frozen eggs. Articles about adjacent topics, often criticizing companies for offering egg freezing as a perk, give a needlessly pessimistic tone: ‘the odds are stacked heavily against you.’ (Glamour magazine); ‘Success rates for frozen eggs vary widely, but rarely go above 30 per cent’ (Financial Times); ‘Despite the growing popularity of egg freezing for women who want kids eventually but not right now, doctors don’t actually recommend the procedure for this purpose’ (Business Insider).

These articles usually reference a study from 2022 that finds that only 39 percent of patients had a baby, but what this coverage misses is that the average age of the women was 38 when they froze their eggs. Their fertility had already begun a precipitous decline. But declining fertility in women is largely about egg, not womb, aging, meaning that a woman who freezes her eggs in her twenties will have roughly the same chances of successful IVF in her forties. The oldest woman ever to give birth using a young donor egg was 74.

Anti-egg freezing media has real life consequences. Women, in general, do not know how successful egg freezing is. Even the women who undergo egg freezing don’t know quite how good it is. Even medical students and doctors don’t know.

Rather than freezing their eggs in their twenties, when it would be optimal, women often freeze their eggs as a last resort. The rate of egg freezing for women aged 18–34 is the same as that for women between 43 and 44. This is despite the fact that women aged 43 to 44 who use their own eggs have a success rate of less than ten percent even after multiple rounds of IVF, versus about 85 percent for women under 35.

Ultimately, the choice of whether to freeze eggs is personal and will depend on finances, life plans, relationship status, and career choices. Like any medical procedure, it is not guaranteed to succeed. Some women are infertile or just very unlucky. But it is possible for most women and much easier than the press would have you believe, as long as it is done in time.

We are both women in our late twenties. We are freezing our eggs to make sure we can have the number of children we want whatever happens in our lives and careers. One of us is a biologist (Ruxandra) and the other is an economist (Luzia). We have teamed up to demystify a process that is both emotionally charged and improperly covered, and explain why more young women should consider freezing their eggs too.

Why fertility falls with age

Why does the ability to get pregnant decline with age? The conventional wisdom is that fertility is relatively constant and then drops sharply after 35. Accurately measuring this is challenging as many studies use small samples or adversely selected populations, like those undergoing fertility treatment, but recent research shows that reality is more complicated than the conventional wisdom might suggest.

One issue is the confusion between fertility and fecundability. Fertility refers to the observed number of births per woman in a population over a given period, while fecundability is the monthly probability that a woman who has regular unprotected sex will conceive a viable pregnancy. Fecundability reflects the biological capacity for conception, whereas fertility reflects the actual number of children a woman has chosen to have.

Historically, researchers could observe only fertility. One of the most consequential studies in this regard comes from 1986, when researchers analyzed a mix of historical populations (including seventeenth-century Genevan bourgeoisie and twentieth-century Iranian peasants) and produced an age-fertility curve that became the foundation for modern beliefs about age-related reproductive decline. Their figure showed a concave pattern, in which fertility was stable through a woman’s twenties and early thirties before dropping sharply after the age of 35.

However, this fertility data is a poor proxy for fecundability. Many of the younger women in these historical populations were biologically incapable of conceiving, either because they were pregnant or because they had recently given birth. Including these women in the denominator artificially depressed fecundability estimates for younger women. Older women, who were less likely to be pregnant or postpartum, did not suffer the same bias.

Modern data shows that the cliff is a statistical illusion. Once you restrict the denominator to women who can actually conceive, fecundity declines roughly linearly from the early twenties onward, with no abrupt mid-thirties break. But it does decline. This decline in fertility with age is underpinned by two factors: a decrease in the quantity of eggs and a decrease in the quality of eggs.

Running out of eggs

Women are born with all the eggs they will ever have, each sitting in a protective sac called a follicle. Most of the eggs are immature, kept in the spare pool called the ovarian reserve. Only a small number of them grow to become antral follicles. These antral follicles are the eggs that the body will mature and release during ovulation. They will either be fertilized or removed in a menstrual cycle. They are also the eggs that can be recovered in an egg freezing cycle.

Women can be born with anywhere from two to six million eggs in their ovarian reserve, but most will ovulate only three to four hundred over a lifetime. The vast majority of primordial follicles are instead destroyed through a quality control filter that eliminates follicles whose eggs or surrounding cells are metabolically stressed, damaged, or poorly supported.

This steady attrition drastically reduces the ovarian reserve over time. By the time a woman hits menopause, only a thousand of the original two to six million remain. As the ovarian reserve declines, the antral follicle count declines almost proportionally. There is a strong correlation between antral follicle count and age. This relationship helps explain why IVF success rates decrease with age.

Egg quality

Egg quality declines with age too. As she ages, a woman’s eggs lose their ability to maintain the correct number of chromosomes.

Unlike most body cells, which routinely divide and make new proteins, eggs spend decades in a hibernation-like state that lasts until they are ovulated. Protein turnover is minimal during this period. Although eggs live for a remarkably long time compared to other cells, this hibernation does not stop all the effects of aging: oxidation, DNA damage, epigenetic drift and mitochondrial dysfunction all still happen. As a result, the proteins that run cell division become damaged or depleted, and the molecular pieces needed to guarantee the right number of chromosomes start to fail.

The cohesin complex is a ring of proteins that clamps each pair of chromosomes together until the moment they should separate. When the cohesin complex ages, the clamp degrades, potentially leading to a premature separation of the chromosomes.

Centromeres, the protein docking sites on chromosomes, also lose structural integrity with age. Normally, these form sturdy attachments to spindle microtubules, the protein cables that move chromosomes during cell division. When they weaken, chromosomes are more likely to attach improperly or be pulled in the wrong direction.

Together, weakened cohesion and faulty attachments cause chromosomes to split unevenly. This means more eggs end up aneuploid (with too many or too few chromosomes) and a smaller share remain euploid (with the normal set). Aneuploid eggs give rise to aneuploid embryos, which usually fail to implant, or result in early miscarriage. In the rare cases that aneuploid embryos can be carried to term, they grow into people with severe developmental disorders such as Down syndrome (three copies of chromosome 21, called trisomy 21), Patau syndrome (trisomy 13), and Edwards syndrome (trisomy 18), all of which increase in incidence with maternal age, sharply after 28.

The egg freezing and IVF journey

Before starting an IVF cycle, women undergo a series of medical evaluations, including blood tests and hormone measurements. The baseline transvaginal ultrasound measures the antral follicle count, the best estimate of how many eggs can be retrieved, and in turn the best indicator of IVF success that we have, given a woman’s age and health.

The actual IVF cycle begins with a drug regimen that induces multiple follicles to mature over 10–12 days. During this time, the patient undergoes ultrasound monitoring every two or three days to track follicle growth. Once the follicles reach the desired size, they are collected through a minor surgery. Roughly four fifths of all follicles removed yield eggs, and roughly four fifths of those eggs are suitable for fertilization. The suitable eggs are then inseminated and cultured for about a week before they are ready for transfer. This stage suffers from high attrition, and only about half of the fertilized eggs complete it. Of these, one is typically selected for transfer to the uterus in the hope of achieving a pregnancy and live birth.

​​Egg or embryo freezing follows the same course, except the process either stops after the eggs have been retrieved or after embryos have formed. The collected eggs or embryos are instead frozen and stored until the patient is ready to get pregnant. Historically, this relied on slow freezing, a gradual cooling method that took several hours and often formed damaging ice crystals inside the cells. Since the mid-2000s, a rapid flash freezing technique, which cools eggs to minus 196 degrees Celsius almost instantly, has been used instead, preventing ice crystal formation and dramatically improving survival rates. Once frozen, the biological clock for these gametes or embryos is effectively stopped. Large-scale studies show that frozen eggs almost always retain their quality and chromosomal integrity, and thawed eggs that survive defrosting perform as well as fresh eggs of the same biological age.

How many eggs can you expect per cycle?

Women between 18 and 34 in developed countries undergoing embryo freezing cycles collect a median of 14 eggs per cycle. There is a lot of natural variation. A cycle can yield anything from zero to more than 50 eggs, even among women at peak fertility. But since roughly 80 percent of a woman’s antral follicles are retrieved as eggs, knowing how many antral follicles a woman has allows us to predict how many eggs a cycle will yield accurately.

This number is whittled down by the embryo stage. As in normal IVF, half of fertilized eggs become suitable for transfer, and two thirds of these have the right number of chromosomes. Ultimately, for the median egg yield of 14 eggs you can expect 2.9 healthy frozen embryos on average, or to put it another way: young women who go through even one cycle of embryo freezing can often freeze enough euploid embryos to have a majority chance of a live birth.

The numbers are even more optimistic for women in their twenties. Data on egg freezing is often combined across the 18–34 age range, which can lead to an underestimate of the number of eggs retrieved for younger women in that group. The largest study on egg donation programs, carried out on 9,539 women with a median age of 26, reported a median of 20 retrieved eggs and six usable embryos.

The practicalities of egg freezing

Having outlined how egg freezing works and the probabilities that determine its success, we now turn to the main practical considerations that shape how women pursue it: cost, location, and clinic choice

Many women assume egg freezing is prohibitively expensive because they are exposed to American prices ($17,000 per round) or, to a lesser extent, the UK (where it costs around $8,500). Egg freezing at a Spanish clinic uses the same state-of-the-art techniques and equipment, but costs only around $5,000. Spain is internationally recognized for its excellent IVF and has been a pioneer in adopting innovative reproductive technologies. For example, IVI Valencia was among the first clinics to adopt vitrification for egg freezing and published research on its success rates, helping to establish the technique in everyday fertility practice.

The full egg-freezing process takes about two to three weeks, but many Spanish clinics partner with clinics in the UK and other countries so that all pre-retrieval monitoring can be done locally. In that case, you need to travel to Spain only for the egg retrieval, requiring just a four-day stay, and then again for IVF some years later. For every year you wait, you will likely save several hundred dollars, as storing frozen eggs costs about $500–$1,000 in the US and $200–$500 in Spain. At worst, the price difference pays for two trips to Spain. At best, you will save several thousand dollars on top.

A second major issue is clinic quality, which varies dramatically. For example, low oxygen levels are important in embryo culture because early embryos are not adapted to the high oxygen concentration found in room air. In the body, mammal embryos develop in the fallopian tube and uterus at oxygen concentrations of around two to eight percent, far lower than the 21 percent oxygen of atmospheric air. Yet many clinics still fail to use natural oxygen levels in their embryo culture chambers.

These differences matter: success rates are not uniform across the industry. The top clinics achieve success rates above 60 percent per embryo transfer, more than double those of the lowest-performing clinics, which are closer to 30 percent. Verifying your clinic’s success rate is extremely important: we recommend the CDC website in the US; the UK fertility regulator’s Choose your clinic tool; and the Spanish Reproduccion Asistida ORG.

Clinics do not all use the same metrics and the variation is often confusing. The three most meaningful metrics are success per embryo transfer, which measures the number of live births once an embryo is transferred, success per IVF cycle, which measures live births from the start of an egg retrieval cycle, and frozen egg survival rate, which is the proportion of eggs that survive thawing years later.

We recommend relying on statistics from official agencies rather than the headline numbers advertised by clinics themselves. Agencies across countries report success per embryo transfer and per IVF cycle. No regulatory body requires fertility clinics to report frozen egg survival rates, despite this being a fundamental determinant of whether egg freezing ultimately pays off. While overall clinic quality is correlated with egg-freezing success, patients should look for more specific indicators. Positive signs include clinics that have published peer-reviewed research on their egg-freezing programs, clinics with large and well-established egg donation programs, and clinics with long-standing experience in egg freezing.

Should you freeze your eggs?

Egg freezing’s problem today is opacity, not ineffectiveness. We are freezing our eggs. If you are a woman in your twenties or early thirties who can afford it, and think you are very likely to want to have children, but are not sure exactly when, then we think you should freeze yours too. If you face expensive British or American prices, go on holiday to Spain, where it is equally state of the art but many times cheaper. If you definitely do not want to have children, or want only one and have a partner lined up already, then you may not need to bother.

To achieve a strong chance of a future live birth, we recommend freezing approximately 20 eggs in your late twenties or early thirties. This estimate is based on interpolating across the largest available studies, though the literature is not fully consistent. For example, a 2016 study from Spain found that for women under 35, the cumulative live birth rate plateaued after about 15 frozen eggs, reaching 85.2 percent, with a 95 percent confidence interval between 60.5 and 100 percent. Other studies suggest a slightly higher cutoff, closer to 20 eggs. When donor eggs are used, where the donors are selected for high fertility and health, the cumulative success rates reach almost 100 percent.

Whatever you do, do not listen to the articles that say that egg and embryo freezing do not work. Not only do they work, but they work almost as well as the best way to guarantee your fertility, which is to be young when you try to conceive. Egg freezing is the best way to give women the freedom to control when they have children, without having to worry they will not be able to.

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Ruxandra Teslo leads the Clinical Trial Abundance Initiative. You can follow her on Twitter.

Luzia Bruckamp is an economics PhD student at the London School of Economics. You can follow her on Twitter.

Read their guide to all the other practical details of egg freezing here.