Frozen vs fresh embryo transfer — what the current evidence actually favours

For the first decade of widespread IVF practice, fresh embryo transfer was the default. You stimulated, you collected, you fertilised, and you transferred — usually on day three or five of embryo development, while still in the same cycle.

That model has changed substantially. Frozen embryo transfer (FET) now accounts for the majority of transfers in many clinics, including most UK fertility centres. The question worth asking is: why, and what does the evidence actually say?

What the large trials show

Several large randomised controlled trials have now compared fresh and frozen transfer outcomes. The picture that emerges is nuanced — and worth understanding before assuming one approach is universally superior.

"The evidence doesn't say frozen is always better. It says frozen is better in specific circumstances — and that in others, it makes little measurable difference."

For women who produce a high number of eggs and are at risk of ovarian hyperstimulation syndrome (OHSS), a freeze-all strategy is clearly supported by evidence. Transferring in a subsequent, unstimulated cycle avoids the hormonal environment of a stimulation cycle, which appears to be less favourable for implantation in this group.

Where the evidence is less clear

For normal responders — women who produce a typical number of eggs without significant OHSS risk — the picture is less definitive. A major Nordic trial published in 2024 found no significant difference in live birth rates between fresh and frozen transfer in this population. Other studies have found small advantages for frozen, particularly around singleton birth rates and reduced obstetric complications.

What to ask your clinic

The honest answer is that the right approach depends on your individual response to stimulation, your progesterone level on trigger day, and your clinic's outcomes data for each approach. These are exactly the questions worth raising at your next appointment.