Fresh or frozen transfer? What the freeze-all approach means

In a classic cycle the embryo is transferred three to five days after egg collection, while the body is still under the influence of the high hormone levels from stimulation. In a freeze-all approach, all embryos of sufficient quality are vitrified and the transfer takes place in a later cycle, when the endometrium and the hormones have returned to a more natural state. Vitrification made this possible: embryo survival after thawing is now very high, and frozen embryos are no longer a second-best option.

That is precisely why the share of frozen transfers rose so quickly in Europe. Separating stimulation from transfer lets the team choose the best moment for the endometrium, avoid high-risk cycles, and wait for test results when those are part of the plan. For you it means one more step in the calendar, but often a calmer cycle and a well-planned transfer.

The clearest situation is a high response to stimulation, with many follicles and a high oestradiol, where the risk of ovarian hyperstimulation syndrome is real. In these cases a GnRH agonist trigger is used and all embryos are frozen; that combination greatly reduces the risk of severe OHSS, because a fresh pregnancy would prolong and worsen it. Freeze-all is also built into progestin-primed protocols, known as PPOS, where a fresh transfer is not possible at all.

There are other reasons. When genetic testing of embryos, PGT-A, is planned, the results arrive after the biopsy and the embryos wait frozen. When progesterone in the blood is high on the day of the trigger injection, the endometrium has advanced too far and is no longer in step with the embryo; freezing solves this. Finally, endometrial reasons such as a polyp found late, a thin lining or fluid in the uterine cavity make freezing the most sensible choice.

Freeze-all is not better for everyone. The ESHRE guideline on ovarian stimulation, in its 2025 update, does not recommend routinely freezing all embryos for every patient. In normal responders, without OHSS risk, without high progesterone and without endometrial reasons, a fresh transfer gives similar live birth rates. Large studies have shown this clearly: the advantage of freeze-all appears in high responders, where the live birth rate is better and OHSS rarer, while in others the difference is small.

There is another side that is worth knowing. Pregnancies after a frozen transfer carry a somewhat higher risk of babies large for gestational age and of hypertensive disorders in pregnancy; pregnancies after a fresh transfer carry a somewhat higher risk of small babies and of preterm birth. Neither route is free of trade-offs. And in practical terms, freeze-all lengthens the time to pregnancy a little, because one more cycle is added.

If a frozen transfer is chosen, there are two ways to prepare the endometrium. In a programmed cycle with oestrogen and progesterone, the transfer date is easy to set, but the body does not form a corpus luteum. In a natural or modified natural cycle, the transfer is timed to your own ovulation. The natural cycle is increasingly preferred in women who ovulate regularly, because programmed cycles are associated with higher rates of hypertensive disorders in pregnancy. For women with irregular cycles, the programmed cycle remains the practical option.

If the team proposes freeze-all, read it as a plan and not as bad news. Your embryos are there, stored in a state where their quality does not change, and the transfer will take place under the best conditions. The right question is not “why did we not do it fresh”, but “why this route for me, and when do we plan the transfer”. That short conversation gives the whole calendar its meaning.