Endometrium

The endometrium is the lining of the uterus, the layer that thickens across the month and is shed as a period if no pregnancy arrives. In treatment it is usually called the lining, and it is measured on ultrasound before an embryo is put back.

Where you'll meet it

At the scans before a transfer. Someone reads out a number in millimetres and a description of the pattern, and the transfer either goes ahead or is postponed on the strength of it. It comes up most in a frozen embryo transfer, where preparing the lining is most of what the cycle consists of.

What the lining does

Oestrogen thickens it through the first half of the cycle. After ovulation, progesterone changes its character, turning it from a growing layer into a receptive one, ready to accept an embryo. That second change is what people mean by the window of implantation, and its timing is why progesterone support is such a fixture of treatment.

In a medicated transfer cycle both hormones are given as drugs. Oestrogen as tablets or patches to build the lining, then progesterone as pessaries, injections or gel to open the window, with the transfer timed from the day progesterone started. In a natural cycle transfer the clinic tracks your own ovulation instead and times the transfer to that, usually with some progesterone support anyway.

The number on the scan

Thickness is measured on ultrasound, where a healthy lining often shows as a triple line, and most clinics want it somewhere around seven or eight millimetres before proceeding. Those thresholds are conventions rather than hard biology. Pregnancies happen below them and fail above them, and no single figure is a perfect cut-off.

A thin lining is the common problem, and the usual responses are more oestrogen, a longer build-up, or switching to a natural cycle. A persistently thin lining is worth investigating rather than repeatedly medicating, because scarring inside the cavity from a previous procedure or infection is one cause, and a hysteroscopy can look for it.

Structural problems sit alongside thickness. Polyps, fibroids pressing into the cavity, and adhesions all get found on scans or a saline study and are generally dealt with before a transfer rather than after a failure.

Scratching and receptivity testing

Two add-ons are offered around the lining, and both are worth weighing carefully.

Endometrial scratching, a deliberate small injury to the lining in the cycle before treatment, was popular for years on the theory that it improved implantation. Larger trials have not supported it, and it is now much less widely recommended.

Endometrial receptivity testing takes a biopsy to work out whether your window of implantation sits at the standard time or is shifted, so the transfer can be moved to match. The idea is sound, though the evidence that it produces more babies is still limited. If either is offered to you, ask what it would change about your plan and what the trial evidence actually shows.

Questions to ask

Ask what your lining measured and what the clinic's threshold is. Ask whether they prefer a medicated or a natural cycle for you, and why. If a transfer has been cancelled for a thin lining more than once, ask whether it is worth looking inside the cavity rather than adjusting the dose again.

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