What is the optimal timing and strategy for frozen embryo transfer in different patient populations?

[Tier B — the ESHRE 2026 ovarian stimulation guideline covers stimulation, not endometrial preparation for FET] What is verified, from ESHRE 2026 on the freeze-all decision: A freeze-all strategy is recommended to minimise the risk of late-onset OHSS (strong). A GnRH agonist trigger combined with freeze-all is recommended in women at risk of OHSS (strong). It is probably recommended to measure serum progesterone on the day of final oocyte maturation in cycles aimed at fresh transfer (conditional), with a good practice point that where progesterone is high the patient should be counselled about potentially lower ongoing pregnancy and live birth rates, and that the decision to defer transfer should also weigh oocyte number, embryo number and embryo quality. Where freeze-all is planned, progestin for pituitary suppression is probably equally recommended to GnRH analogues (conditional). On endometrial preparation itself — natural, modified natural or programmed cycle — this guideline does not cover it, and I have no verified source. The main considerations are scheduling flexibility versus the need for luteal support in programmed cycles, where inadequate support is a recognised and correctable cause of failure. On the interval after retrieval, I could not verify a recommendation and am not going to state one. Reviewer: this answer needs a source on endometrial preparation protocol and on timing of the first FET after retrieval. Both are areas of genuine practice variation and the answer currently reflects that honestly rather than asserting a standard.