Your embryo can wait. Your lining gets ready first.

A frozen embryo transfer is when an embryo frozen after IVF is thawed and placed in the womb in a later cycle.

Embryos are frozen by vitrification, a fast method that avoids ice damage. Your lining is prepared in the transfer cycle, either with medicines or by following your own ovulation. On the day, the embryo is thawed in our lab and placed in the womb with a soft catheter, guided by ultrasound. It takes a few minutes and needs no anaesthesia.

A frozen embryo transfer is usually done around day 18 to 22 of the cycle, counting day 1 as the first day of your period, but the exact day is set by your lining and progesterone, not the calendar.

In a medicated (HRT) cycle, you start oestrogen tablets on day 2 or 3. A scan about 10 days later checks the lining. Progesterone then starts, and a blastocyst (day-5 embryo) goes in on the 6th day of progesterone; a day-3 embryo on the 4th day. If the lining needs longer, the transfer moves later. In a natural or letrozole cycle, we track your follicle and ovulation. A blastocyst goes in about 5 days after ovulation, usually around day 19 to 21 in a regular 28-day cycle, later if your cycles are longer.

Neither is better for everyone: a medicated cycle gives a date you can plan around, and a natural cycle uses fewer medicines but needs regular ovulation.

We use the medicated (hormone replacement) method most often, because it works whether or not you ovulate and lets us fix the date. Oestrogen also stops your own ovulation that month. The letrozole method follows your own cycle and suits women who ovulate regularly. Research comparing the two is still going on, so the choice is made for your cycle, not by rule.

We freeze most embryos because the lining can be prepared under calmer, more controlled conditions than during stimulation.

Stimulation injections push oestrogen, and sometimes progesterone, well above normal, which in some women may affect how ready the lining is. Freezing all embryos protects women at risk of over-response (OHSS), such as some with PCOS, and is needed when embryos are tested with PGT-A. A fresh transfer is still an option when response, hormones and lining are all favourable.

If the lining is too thin or progesterone rises too early, the transfer is moved or the cycle is stopped, and your embryo stays safely frozen.

We usually look for a lining over 8 mm on the scan, though pattern matters too. Before starting progesterone, a blood test checks that progesterone is still low (under 1 ng/ml). Sometimes a woman ovulates despite the tablets; then we cancel rather than transfer at the wrong time. It is disappointing. It also protects the embryo.