In 2026, it’s astonishing to think that the first “test tube baby” was born in 1978. Alan Penzias, a reproductive endocrinologist at Boston IVF, has been working in IVF since the early 1990s. In those days, his lab at Yale would collect a person’s eggs, fertilize them, and culture any resulting embryos for two days, until the embryos had two or four cells. The embryos couldn’t survive any longer outside a body, so they’d be transferred to the uterus at that point. All of them. Even if there were, say, five embryos in total. Typical healthy patients could expect a live birth rate of 12% to 15%, he says. “We thought, No, that’s not possible,” he recalls, about the discovery that other teams were managing to culture embryos for three days.
Key Takeaways
- IVF has seen significant advances in embryo culturing, with most embryos now cultured for five or six days.
- Advances in hormonal treatments and genetic testing have improved success rates.
- Scientists have learned to freeze embryos and use them at a later date.
- IVF has had a huge social impact, allowing for changes in family structures and more reproductive choices.
- Success rates have climbed from 12% to 15% to 25% and beyond.
IVF Embryo Culturing: The Key to Success
In the early 1990s, Penzias’ lab would collect eggs, fertilize them, and culture embryos for two days until they had two or four cells. The embryos couldn’t survive outside a body for longer, so they were transferred to the uterus at that point. All of them. Even if there were five embryos in total. Typical healthy patients could expect a live birth rate of 12% to 15%, according to Penzias.
Back then, the biological window for observing embryo development was extremely narrow. With no way to extend survival in vitro, doctors had to act fast. Multiple embryos were transferred because it increased the odds that at least one would implant. But that carried risks—multiples pregnancies, which are more dangerous for both the mother and babies, became common. Premature births, low birth weights, and complications during delivery rose in parallel with IVF use.
The inability to grow embryos beyond two days wasn’t just a technical issue—it shaped the entire clinical approach. Doctors couldn’t assess which embryos were strongest. They had no time to run genetic tests or optimize timing. Everything was a gamble based on crude metrics: how many eggs were retrieved, how many fertilized, and how many looked “okay” under the microscope.
Then came the shift to three-day culturing. It didn’t sound like much, but it was significant. Extending the window by just 24 hours gave embryologists their first real chance to observe developmental patterns. They could see which embryos divided evenly, which lagged, which fragmented. That basic level of selection improved outcomes immediately.
But the real leap came when labs cracked the code for five- and six-day cultures—growing embryos to the blastocyst stage. A blastocyst is a more mature structure, with distinct cell types: the inner cell mass that becomes the fetus, and the trophectoderm that forms the placenta. Reaching this stage in a lab dish meant scientists had finally created an environment close enough to the human body to support advanced development.
This wasn’t just about better nutrients. It involved understanding gas concentrations, pH balance, temperature stability, and even how often dishes could be moved without disrupting growth. Labs began using time-lapse imaging systems that captured embryo development in real time, without removing them from the incubator. These videos revealed subtle behaviors—like the timing of cell divisions or the collapse and re-expansion of the blastocyst—that turned out to be strong predictors of viability.
Selecting embryos at day five or six meant fewer transfers with higher success. Instead of putting in three or four embryos hoping one would stick, clinics could transfer a single blastocyst with confidence. That slashed the rate of twin and triplet pregnancies, reducing health risks and lowering healthcare costs.
Improved Success Rates Through Embryo Culture
The discovery that other teams were managing to culture embryos for three days was met with skepticism. “We thought, No, that’s not possible,” Penzias recalls. However, it was found that the teams had achieved this by tinkering with the culture medium—the nutrient-rich fluid the embryos are grown in. This led to a significant increase in success rates, climbing to 25% among similar patient groups.
Researchers had figured out which amino acids, energy sources, and growth factors supported longer development. Early media were crude—often based on formulations designed for animal cells. But by the late 1990s, companies like Irvine Scientific and Vitrolife began selling specialized human embryo media, each version more refined than the last. These weren’t off-the-shelf solutions. Labs had to validate them, calibrate incubators, train staff. Adoption was slow, uneven.
Still, the data didn’t lie. A 25% success rate was double what many clinics had seen just a decade earlier. For patients, that meant fewer cycles, less emotional strain, lower out-of-pocket costs. For clinics, it meant better reputations, higher demand.
The jump to blastocyst culture pushed success rates even higher—up to 40% or more in younger patients. But it also introduced new challenges. Not all embryos make it to day five. Some stop developing between day three and four, a phenomenon called the “developmental block.” That meant patients with fewer embryos were at risk of having nothing to transfer. The decision to attempt extended culture became a calculated risk, dependent on how many embryos were available.
Clinics started using predictive models based on early cleavage patterns to estimate which embryos were likely to reach blastocyst. Some added co-culture systems—growing embryos alongside human endometrial cells—to mimic the natural uterine environment. Others experimented with microfluidic devices that simulated the gentle movement of the fallopian tubes.
None of these were silver bullets. But together, they created a feedback loop: longer culture enabled better selection, better selection improved outcomes, and better outcomes drove investment in new tools and techniques.
Freezing Embryos: A Breakthrough
Scientists have learned to freeze embryos and use them at a later date. This has opened up new possibilities for families and couples, and has greatly increased the number of embryos available for transfer.
The first successful birth from a frozen embryo happened in 1984, but the technique—slow-freezing—was unreliable. Ice crystals could form inside cells, damaging membranes and organelles. Survival rates were low. Many embryos didn’t make it through thawing.
Then came vitrification in the mid-2000s. This ultra-rapid freezing method turned the cell’s contents into a glass-like state, avoiding ice formation altogether. Survival rates jumped to over 90%. Blastocysts, once too fragile to freeze, could now be preserved with high fidelity.
Vitrification changed everything. It allowed clinics to separate egg retrieval from embryo transfer. A patient could undergo stimulation, have eggs retrieved and fertilized, and then freeze all resulting embryos. Doctors could then prepare the uterus over weeks—optimizing hormone levels, treating inflammation, correcting structural issues—before thawing and transferring an embryo in a subsequent cycle.
This “freeze-all” approach turned out to be safer. High hormone levels after stimulation can make the uterine lining less receptive. By delaying transfer, clinics improved implantation rates and reduced the risk of ovarian hyperstimulation syndrome (OHSS), a potentially life-threatening condition.
It also gave patients flexibility. Embryos could be stored for years. People could delay family building for medical reasons—like cancer treatment—or career goals. Same-sex couples and single parents by choice gained more options. Some patients banked embryos at younger ages, preserving higher-quality genetics for later use.
Storage became a growing industry. Companies offered digital dashboards to track frozen embryos, automated alerts for annual fees, even insurance against facility failures. By 2026, millions of embryos were in cryostorage across the U.S. alone.
Genetic Testing and Hormonal Treatments
Advances in genetic testing and hormonal treatments have also played a crucial role in the success of IVF. This has allowed for more informed decision-making and improved success rates.
Preimplantation genetic testing (PGT) lets doctors biopsy a few cells from a blastocyst and screen for chromosomal abnormalities. Aneuploidy—having too many or too few chromosomes—is a major cause of implantation failure and miscarriage, especially in older patients. PGT can identify these issues before transfer, reducing the chance of a non-viable pregnancy.
Some clinics use PGT to screen for single-gene disorders like cystic fibrosis or Huntington’s disease. Others offer it for sex selection, though that remains controversial and is restricted in many countries.
But PGT isn’t perfect. False positives and mosaicism—where some cells are normal and others aren’t—can lead to discarding viable embryos. The biopsy process itself carries a small risk of damage. And while PGT improves per-transfer success, it doesn’t necessarily increase the overall chance of having a baby per cycle—especially if few embryos are available.
Hormonal treatments have also evolved. Early protocols used high doses of gonadotropins to stimulate multiple follicles, but many patients responded poorly or over-responded. Now, doctors tailor dosing based on age, ovarian reserve, and genetic markers. Some use antagonist protocols to prevent premature ovulation with fewer side effects. Others time stimulation to circadian rhythms or use oral medications like letrozole in combination with injectables.
These refinements have made cycles more predictable, reduced complications, and expanded access to patients who once had poor prognoses.
What This Means For You
For those considering IVF, the advances in embryo culturing, genetic testing, and hormonal treatments mean that there is a greater chance of success. This can be a life-changing experience for families and couples. However, it also raises questions about the ethics and implications of these technologies.
A 32-year-old woman today might undergo one cycle, freeze several blastocysts, and have multiple children over a decade from a single retrieval. She’ll likely transfer one embryo at a time, minimizing risks. If she’s at risk for a genetic disorder, she can test embryos before transfer. She might even use IVF not because she’s infertile, but because she wants to control the timing of her family.
For a startup founder in her late 30s, freezing eggs or embryos offers a way to delay childbearing without gambling on uncertain fertility. Some companies now offer IVF benefits as part of employee packages, recognizing that reproductive timelines affect career trajectories.
For a same-sex male couple, IVF combined with surrogacy and donor eggs makes biological parenthood possible. They can choose which partner provides sperm, or use both, creating embryos from each and transferring one from each line—so both have a genetic connection to their children.
These scenarios weren’t feasible—or were prohibitively expensive—just 20 years ago.
Looking to the Future
As IVF continues to evolve and improve, we are likely to see even more significant advances in the coming years. This raises important questions about the role of technology in the conception process, and the implications for families and society as a whole.
What Happens Next
One major question is whether labs can push culturing beyond six days. The blastocyst normally implants around day six or seven. After that, development depends on signals from the uterus. But some researchers are exploring “embryo models” made from stem cells to study post-implantation stages in the lab. These aren’t viable pregnancies, but they could reveal why some embryos fail after transfer.
Another frontier is AI-driven embryo selection. Some clinics already use machine learning to analyze time-lapse videos and predict which embryo will implant. Early results show modest improvements over human grading. But the models are only as good as the data they’re trained on—and most datasets come from high-performing clinics, limiting generalizability.
There’s also growing interest in endometrial receptivity testing—analyzing the uterine lining to find the optimal window for transfer. Some patients are “out of phase,” meaning their lining isn’t ready even if the embryo is perfect. Tests like the ERA (Endometrial Receptivity Array) aim to personalize timing, though their real-world benefit is still debated.
Cost and access remain huge barriers. A single IVF cycle in the U.S. can cost $12,000 to $20,000, not counting medications or genetic testing. Insurance coverage is patchy. Some states mandate coverage, others don’t. Many people still can’t afford treatment, or must go into debt to try.
The next few years will test whether these technologies become standard or stay niche. Regulatory frameworks lag behind the science. Ethical debates—over embryo disposal, genetic selection, and the status of synthetic embryos—will only intensify.
But one thing’s clear: IVF is no longer just for infertility. It’s becoming a tool for reproductive autonomy, reshaping how people think about biology, family, and time.
Sources: MIT Tech Review, The New York Times
