For years, fresh embryo transfers were standard. You’d do an egg retrieval, grow the embryos, and transfer one a few days later. Freezing was for leftovers.
Now the pendulum has swung. Frozen embryo transfers (FET) have higher success rates for many patients — especially those at risk for ovarian hyperstimulation syndrome (OHSS), those with high estrogen levels, or those using donor eggs. Some clinics now freeze all embryos by default and transfer in a subsequent cycle.
Why the shift? Controlled ovarian stimulation — necessary for fresh transfers — elevates estrogen and progesterone levels, which may make the uterine lining less receptive. The high hormone environment can also trigger OHSS, a painful and potentially dangerous condition. By freezing all embryos and allowing the ovaries to return to baseline, the uterus can be prepared in a more natural or programmed cycle without those super-physiologic hormone levels.
A 2023 New England Journal of Medicine trial randomized over 1,500 patients to fresh vs. frozen transfer. The frozen group had significantly higher live birth rates (about 50% vs. 42%) and lower rates of OHSS. The benefit was especially pronounced in women with polycystic ovary syndrome (PCOS) or those with high egg counts.
But frozen isn’t always better. A different trial in the same year found that in women with normal ovarian reserve and regular cycles, fresh and frozen had similar outcomes. The key is individualization.
Here’s what the frozen transfer process looks like: after egg retrieval and fertilization, all embryos are biopsied (for genetic testing if desired) and frozen. About 4-6 weeks later, you start a “preparation cycle.” Options include a natural cycle (using your own ovulation) or a medicated cycle (using estrogen and progesterone to build the uterine lining). The embryo is thawed and transferred when the lining reaches optimal thickness.
I had a patient — “Nina” — who did a fresh transfer first. She developed OHSS, was hospitalized for two days, and the transfer failed. For her second cycle, her REI recommended a freeze-all and FET. She was nervous about the wait, but the FET was successful. She told me, “The extra month was hard, but my body felt so much better going into the FET. I wasn’t bloated or miserable.”
Another patient, “Claire,” did a freeze-all for genetic testing. Her embryos were biopsied and frozen while the PGT-A results came back. She transferred a normal embryo in a subsequent cycle and conceived. “If I’d done a fresh transfer, I wouldn’t have known which embryos were genetically normal,” she said. “It would have been a gamble.”
Some patients worry about embryo survival through freezing and thawing. The good news: vitrification (flash-freezing) has survival rates above 95% for blastocysts. Losing an embryo is rare. And studies show no increased risk of birth defects or developmental issues with frozen versus fresh embryos.
So is frozen always better? No. For patients with low ovarian reserve or those who need a quick transfer due to cancer treatment, fresh may be preferred. But for many, the freeze-all approach offers higher success rates, lower OHSS risk, and the flexibility to complete genetic testing before transfer.
If you’re starting IVF, ask your clinic about their policy on fresh vs. frozen. Do they freeze all by default? Under what circumstances would they recommend a fresh transfer? What is their FET protocol? Understanding the options will help you make an informed choice.
FET does require patience. The waiting between retrieval and transfer can feel interminable — a month or more of extra anticipation. But that waiting may improve your odds. For many, it’s worth it.
This article reflects 2026 evidence. Always discuss transfer types with your REI.
— Emily Carter, RDN