Frozen vs. Fresh Embryo Transfers: What the Latest Research Shows

A decade ago, roughly 30% of embryo transfers in the United States used frozen embryos. Today, that number is above 75%. According to SART national outcome data, the frozen embryo transfer (FET) has become the default in many clinics, quietly overtaking the fresh transfer as the standard approach. If you are currently in or preparing for an IVF cycle, there is a good chance your doctor has already recommended freezing all your embryos before transferring any of them.

But here is the thing: the reasons behind that shift are not always the ones patients hear about. The latest research suggests the answer to "which is better?" depends on who you are, not just which approach is trendier.

Below, we look at what the evidence actually says about frozen versus fresh embryo transfers, where the two approaches diverge in meaningful ways, and what questions are worth raising with your reproductive endocrinologist before your next cycle.

Three things converged to make frozen transfers dominant. First, vitrification, a flash-freezing technique, dramatically improved embryo survival rates after thawing. The Cleveland Clinic reports that modern vitrification yields survival rates of 95 to 98 percent, a far cry from the slow-freeze era when losing embryos in the thaw was a real concern.

Second, the rise of preimplantation genetic testing for aneuploidy (PGT-A) made freezing a practical necessity. Testing an embryo's chromosomes takes time, so embryos have to be frozen while labs process the biopsy. As PGT-A adoption climbed, so did FET rates. (Whether PGT-A itself improves outcomes for all patients is a separate and still-evolving question; a landmark New England Journal of Medicine trial found no improvement in live birth rates with PGT-A in unselected populations.)

Third, freezing all embryos nearly eliminates the risk of ovarian hyperstimulation syndrome (OHSS), a painful and sometimes dangerous complication of ovarian stimulation. When you transfer a fresh embryo and it implants, the pregnancy hormones can worsen OHSS. Separating stimulation from pregnancy by freezing gives the body time to recover. For high responders and patients with polycystic ovary syndrome (PCOS), this alone made freeze-all a compelling option.

This is where the evidence gets more interesting than the headlines suggest. Several large randomized controlled trials have now compared the two approaches head to head.

A 2018 NEJM study of over 2,100 women in their first IVF cycle found live birth rates of 48.7% for frozen transfers versus 50.2% for fresh. The difference was not statistically significant. An earlier NEJM trial specifically in women with PCOS did find FET produced a higher live birth rate (49.3% versus a lower rate in the fresh group), but that advantage was driven by the specific physiology of high responders.

A 2024 analysis in Fertility and Sterility Reports looking at more than 8,000 first-time IVF patients without PGT-A found live birth rates of roughly 44% for fresh and 46% for frozen, again not a clinically meaningful gap. And a large SART registry study of nearly 83,000 cycles found that freeze-all was beneficial for high responders but offered no advantage for intermediate and low responders.

The takeaway from the data: for most patients, the two approaches produce similar pregnancy and live birth rates. The advantages of freezing are real, but they are population-specific, not universal.

Check out this hot take from Dr. Lora Shaine on the benefits of fresh vs. frozen embryo transfers.

Here is where the conversation gets more complicated. A growing body of research has found that pregnancies from frozen embryo transfers, particularly those using programmed (medicated) FET cycles, carry a higher risk of hypertensive disorders including preeclampsia.

A multicenter cohort study published in 2022 found that programmed FET cycles were associated with an increased risk of hypertensive disorders of pregnancy. Research published in Frontiers in Endocrinology in 2023 linked programmed FET to abnormal placental development, which may help explain the mechanism. And a 2024 meta-analysis confirmed that programmed FET was also associated with large-for-gestational-age infants and higher birth weights.

The important nuance: not all FET protocols carry equal risk. Natural cycle and stimulated FET cycles, where the body produces its own progesterone and develops a corpus luteum, appear to have a lower risk profile than programmed cycles where hormones are supplied entirely through medication. The ESHRE 2024 guideline noted that even singleton pregnancies from FET carry a higher risk of low birthweight compared with those from fresh transfers. This is not a reason to panic, but it is a reason to have a detailed conversation with your doctor about which FET preparation method makes sense for you.

Fresh transfers still have clear advantages in certain situations. If your progesterone level is normal after stimulation, if your response to medication was moderate rather than extreme, and if you are not planning PGT-A, a fresh transfer can mean a shorter time to pregnancy. A BMC Medicine analysis found that while cumulative live birth rates were similar between freeze-all and fresh strategies (42.6% versus 41.7%), the time to first pregnancy was dramatically different: a median of 106 days for freeze-all versus 29 days for fresh.

For patients with diminished ovarian reserve or poor response to stimulation, fresh transfers may also be preferable. These patients produce fewer embryos, so there is less advantage to freezing, and every additional step introduces a small (though now very small) risk of embryo loss. A study in PMC focused on women over 40 with poor ovarian response found no benefit to freeze-all in this group.

The evidence points to clear scenarios where freeze-all is the stronger choice. According to a review in Reproductive Sciences and a clinical indications analysis, freeze-all is most clearly supported when you have a high ovarian response with elevated estrogen levels and an OHSS risk, PCOS (where RCT evidence is strongest for FET), elevated progesterone on trigger day (which can impair the uterine lining), or a need for PGT-A or other embryo testing. In any of these situations, the data supports separating the stimulation cycle from the transfer.

If your clinic has recommended freeze-all or a fresh transfer and you want to understand the reasoning, here are evidence-based questions to bring to your next appointment:

The shift toward frozen transfers has been driven by real improvements in technology and real safety benefits for specific populations. But the research also makes something clear: freeze-all is not categorically superior. It is a tool, and like every tool in reproductive medicine, it works best when matched to the patient rather than applied as a blanket policy.

If your doctor has recommended one approach, you are well within your rights to ask why, and to understand how that recommendation connects to your specific numbers, your specific body, and your specific goals. That is not being difficult. That is being informed.

SART National Outcome Tables — Real-time U.S. success rate data for fresh and frozen transfers

ASRM Embryo Transfer Guidance — Clinical guidelines on transfer protocols

ESHRE Guideline: Number of Embryos to Transfer (2024) — European evidence-based guideline with FET-specific findings

Cleveland Clinic: Embryo Freezing (Cryopreservation) — Patient-friendly overview of vitrification and outcomes