Frozen Embryo Transfer: why frozen often beats fresh

For decades, “IVF” meant fresh transfer: stimulate, retrieve, fertilise, transfer in the same cycle. That is changing. More clinics are now defaulting to a freeze-all approach, where embryos are cryopreserved and transferred in a separate, hormonally-prepared cycle. The reasons matter.

What a frozen embryo transfer cycle looks like

The transfer cycle is independent of the stimulation cycle that produced the embryos. Most clinics use a hormonally-prepared cycle: estrogen for two weeks to build the endometrial lining, then progesterone added to prepare the lining for implantation. Transfer happens on a precisely-timed day, usually 5 or 6 days after progesterone starts.

Some patients use a natural-cycle FET, where the body’s own ovulation triggers the lining preparation. Either approach can work; the data on which is superior is still being settled.

Why fresh used to be the default

Early IVF technology produced poor results from frozen embryos. The slow-freezing methods of the 1980s and 90s damaged embryos at high rates. So clinics transferred while everything was still alive in the dish.

What changed: vitrification

Vitrification – ultra-rapid freezing in liquid nitrogen – became standard from around 2010 onwards. Survival rates of vitrified blastocysts are now greater than 95 percent at most good clinics. This single technology shift makes freeze-all viable.

Why frozen often beats fresh now

During stimulation, the body is in a high-estrogen, sometimes high-progesterone state that may make the endometrium less receptive to implantation. Letting the body return to baseline before transfer, in a cleanly-prepared cycle, may improve outcomes – particularly for high responders, PCOS patients, and patients undergoing PGT-A genetic testing (which requires biopsy + freezing anyway).

Live-birth rates per transfer in current published data are roughly equivalent or slightly higher for frozen transfers in many indications. The picture is not unanimous – for low responders or older patients with very few embryos, fresh transfer may still be preferred.

The downsides

An extra cycle of progesterone or estrogen. An additional waiting period. Some additional cost (storage fees, transfer-cycle medications). And a small but real risk that an embryo will not survive the thaw, even with vitrification at 95 percent survival rates.

Questions worth asking your doctor

• Are we planning fresh or frozen transfer for this cycle? Why?
• If frozen, will we use a hormonally-prepared cycle or a natural cycle? What is the success-rate difference at this clinic?
• How many days of progesterone before transfer, and for how long after?
• What is the survival rate of frozen embryos at this lab, and how do we know if mine survived the thaw?

This essay is educational. Every patient’s situation is different – the right plan is shaped in conversation with a fertility specialist who knows the full picture.