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Frozen embryo transfers (FETs) are a key part of the IVF process — and they’re more common than ever. But even with top-tier embryos, implantation doesn’t always happen. So what gives?
A groundbreaking study led by Dr. Margolis at Fertility Specialists Network (FSN) sheds light on a surprising piece of the puzzle: uterine tissue stiffness. Using a cutting-edge ultrasound tool called shear wave elastography (SWE), Dr. Margolis’s team explored whether measuring the uterus itself — beyond hormone levels and lining thickness — could predict which transfers are most likely to work.
Here’s what to know about frozen embryo implantation success rates, how this new tech could predict your chances of a positive pregnancy test, and what really makes a uterus “receptive.”
What are the odds of a frozen embryo transfer being successful?
Frozen embryo transfers are widely used today, with more clinics offering this option over fresh transfers. And for good reason.
FETs using embryos that were frozen and thawed later have varying success rates (a successful pregnancy) depending on the patient’s age and whether the embryo underwent preimplantation genetic testing, according to recent studies. And with more than 80% of treatment cycles in the US involving embryo freezing (AKA embryo cryopreservation), that’s a considerable amount.
These numbers increase when transferring chromosomally normal embryos (euploid), particularly in younger patients. Some clinics report live birth rates of 60% or higher in first-time FETs using euploid embryos.
But even in ideal circumstances, implantation can still fail. That’s what inspired Dr. Margolis to dig deeper.
“I was inspired by a pretty universal challenge in fertility care: even when we transfer a genetically healthy embryo into a uterus that looks ‘normal’ on ultrasound, implantation doesn’t always happen,” said Dr. Margolis.
“That gap — understanding why a great embryo sometimes doesn’t stick — was my starting point.”
How successful is the first time frozen embryo transfer?
First-time success is absolutely possible — especially when the embryo has been tested and the uterus is receptive.
Dr. Margolis’s research focuses on that second factor — the uterine environment: “Our study focused on transfers where all the embryos were top-tier: euploid and well-graded. That let us zero in on the uterus without the embryo quality being a confounding factor.”
And what they found was eye-opening.
“Interestingly, while we allowed different types of endometrial preparation protocols, these did not significantly affect success rates in our analysis. The key difference-makers were what SWE revealed — subtle differences in the uterus that we wouldn't see on a routine ultrasound.”
Shear wave elastography (SWE): A new way to assess uterine readiness
Shear wave elastography (SWE) is an advanced, non-invasive ultrasound technique that measures how stiff or soft uterine tissue is. The theory? The mechanical properties of the uterus might reveal whether it’s “open” to an embryo at the time of transfer — something traditional scans can’t see.
“While we've made big leaps in embryo testing, our tools for assessing uterine readiness haven’t kept pace. So we set out to explore whether a newer ultrasound technology called shear wave elastography (SWE), which measures uterine tissue stiffness, could give us deeper insight into which uteruses are most receptive at the time of embryo transfer,” explains Dr. Margolis.
What did the study find?
Dr. Margolis’s team studied over 150 patients undergoing a frozen embryo transfer of a single, chromosomally normal (euploid) embryo. Using SWE the day before transfer, they measured the stiffness in different regions of the uterus.
“We found that certain patterns of stiffness throughout the uterus were associated with higher chances of pregnancy,” said Dr. Margolis.
“We then built a predictive model that, when combining these ultrasound findings with basic patient info, was over 90% accurate at forecasting whether a patient would become pregnant. That’s a big step forward — imagine being able to better predict (and even one day improve) your transfer outcome with a simple, non-invasive scan.”
Do age, BMI, or past failed transfers affect frozen embryo implantation success?
They can — but maybe not as much as we thought.
“Since we focused on a relatively healthy group — people using single, genetically normal embryos and without obvious uterine problems — we didn’t find major differences in success rates based on age or most demographic factors,” said Dr. Margolis.
“Age at transfer, BMI, and even the number of previous failed transfers weren’t significantly linked to outcomes in this group. That said, this makes our findings even more powerful: even in a group where most people ‘should’ have good outcomes, SWE still identified who was more likely to get pregnant. In the future, we’d love to expand this to more diverse patient populations, including those with uterine anomalies or repeat implantation failure.”
Are frozen embryos more likely to implant?
It might sound counterintuitive, but frozen embryos aren’t necessarily at a disadvantage — in fact, they can sometimes have higher implantation rates than fresh ones, depending on the circumstances.
Freezing gives the body time to recover from ovarian stimulation (the hormonal injections taken before egg retrieval), which can improve uterine receptivity. It also allows clinics to better control the timing of the transfer and ensures hormone levels are optimal.
And as Dr. Margolis’s study shows, timing is only part of the story.
“Timing still matters, of course, but our findings suggest there’s a hidden layer of uterine readiness that goes beyond endometrial thickness and hormone levels. SWE may help us uncover that.”
A meta-analysis published in The BMJ also found that frozen embryo transfers were associated with a higher live birth rate than fresh transfers in people with PCOS. However, for people without PCOS, the difference was smaller and may depend on individual factors.
Do frozen embryos implant slower?
Not exactly. Implantation timing depends on the stage the embryo was frozen (Day 3 vs. Day 5), how the endometrial lining is prepared, and whether the timing mimics a natural cycle.
Most clinics align the embryo and lining to sync up with implantation windows, so even if the embryo was frozen, it’s thawed and transferred in a way that supports immediate implantation.
How do I know if my frozen embryo has implanted?
Ah, the two-week wait. While implantation happens within a few days of transfer, it can be hard to tell until testing day.
Here are some common signs that could suggest implantation:
- Light spotting or cramping around 6–10 days post-transfer
- A slight increase in basal body temperature
- Breast tenderness or changes
- Feeling fatigued or emotional
But it’s also completely normal to feel nothing at all. The only way to know is through a beta hCG blood test about 9–12 days after transfer.
What increases success of frozen embryo transfer?
There’s no one-size-fits-all formula for a successful FET, but there are a few key factors that have been shown to help:
1. Embryo quality
Using a genetically tested (euploid) embryo increases the chance of success, as it removes a major variable from the equation.
2. Uterine receptivity
This is where the new research from Dr. Margolis is making waves. Her team used the novel ultrasound technique — shear wave elastography (SWE) — to assess subtle biomechanical factors that standard ultrasounds can’t detect.
“Our results suggest [SWE] could one day help personalize FET timing or flag patients who may benefit from additional treatment to optimize their uterine environment,” says Dr. Margolis.
3. Endometrial preparation
Most people undergoing FET use either a natural cycle or a medicated protocol to thicken the uterine lining and sync it with the embryo’s stage of development. Interestingly, in Dr. Margolis’s study, the type of protocol didn’t significantly affect success — which shifts the focus back to the uterus itself.
4. Timing
Even with the best embryo, transfer needs to happen within a narrow “implantation window.” That’s why monitoring lining, ovulation (or hormones), and now uterine stiffness matters.
5. Whole-body health
Lifestyle factors like smoking, untreated thyroid issues, chronic inflammation, or PCOS can lower the odds of success. That said, it’s important not to internalize blame — you can do everything “right” and still need a few tries.
What does this mean if you’re planning an FET?
If you’re gearing up for a frozen embryo transfer, here’s what Dr. Margolis recommends:
“Our study offers two big takeaways. First, the uterus matters, and its receptivity may not be fully captured by current evaluations. Second, we now have a promising tool, SWE, that could help better assess uterine readiness and possibly guide treatment decisions.”
While SWE isn’t yet part of routine fertility care everywhere, Dr. Margolis’s findings suggest it could play a major role in the future of IVF.
“It’s a peek into the future of precision fertility care — and it’s exciting!”
Frozen embryo success is about more than just luck
For anyone going through IVF, the phrase "transfer day" can carry both hope and fear. While science has made huge strides in selecting the best embryos, we’ve lagged behind in understanding what makes a uterus ready to receive one.
Thanks to research like Dr. Margolis’s, that’s changing.
Frozen embryo transfers are already effective — but Dr. Margolis’s research shows they could get even better.
With shear wave elastography, clinics may soon be able to predict transfer success with over 90% accuracy, giving patients more confidence and clarity.
And while no test can guarantee a pregnancy, this one offers something powerful: hope backed by real, measurable data.
Tassia O'Callaghan is an experienced content writer and strategist, having written about a vast range of topics from chemical regulations to parenting, for brands like Peanut App Ltd, Scary Mommy, Tally Workspace, and Fertility Mapper. She's an advocate for realistic sustainable living, supporting small businesses (author of A-Z of Marketing for Small Businesses), and equity across all walks of life. Follow her on LinkedIn or TikTok, or see more of her work on Authory or her website.