Thin endometrium: when the lining doesn’t thicken enough

It’s a frustrating situation: the embryos are there, frozen, ready, but the lining isn’t keeping up. Here’s what we know about a thin endometrium, and the options that have actually been evaluated.

How thick does it need to be?

The endometrium is the lining of the womb; it thickens under the influence of oestrogen. Before a transfer, it is measured by ultrasound. Most clinics aim for at least 7 mm, ideally 8 mm or more, with a ‘three-line’ appearance.

A large Canadian study involving over 40,000 frozen embryo transfers showed that the chances decrease gradually below 7 mm, though there is no absolute threshold: pregnancies do occur with a thickness of 5 or 6 mm. Conversely, above 8 to 10 mm, any additional thickness offers no further benefit. Thickness is therefore one factor amongst many, not a verdict.

Causes of a thin endometrium

• Asherman’s syndrome: adhesions within the uterine cavity, often following a curettage, an infection or uterine surgery. This is the cause to investigate first.
• Chronic endometritis: a silent inflammation, diagnosed by biopsy and treated with antibiotics; see hysteroscopy and endometritis.
• Pelvic radiotherapy or repeated surgical procedures.
• Poor response to oestrogen with no identified cause, sometimes linked to insufficient blood supply.
• Effect of certain medicines: clomiphene citrate thins the endometrium in some women.

Diagnostic work-up

A hysteroscopy is the key examination: it provides a direct view of the cavity, allows adhesions to be released and enables a biopsy to be taken. A 3D ultrasound scan or a hysterosonography may be carried out beforehand.

Preparation options

• Adjusting oestrogen therapy: changing the route of administration (oral, patch, gel, vaginal), increasing the dose, or extending the duration. There is no urgency to proceed with the transfer: preparation can be extended by several days.
• Trying a natural cycle: in a woman who ovulates, the endometrium prepared by her own follicle is sometimes of better quality than in a medicated cycle. See the ways a frozen embryo transfer can be prepared.
• Treating the cause: hysteroscopic adhesiolysis in cases of Asherman’s syndrome, antibiotics in cases of endometritis.

Treatments sometimes offered, and the evidence for them

• Low-dose aspirin, vaginal sildenafil, pentoxifylline and vitamin E: used to improve blood flow. The studies are small and of poor quality; some show an increase in endometrial thickness, but there is no solid evidence of an increase in births.
• G-CSF (growth factor injected into the uterus): not recommended by ESHRE outside of research.
• Intrauterine PRP: some encouraging results in small studies, but ESHRE reserves its use for research; see the article on PRP.
• Stem cells: experimental.

These treatments are not necessarily dangerous, but they must not be presented as proven solutions, nor should they delay a transfer indefinitely.

When to transfer anyway?

After several attempts at preparation, many doctors agree to proceed with a transfer at 6 to 7 mm if the lining looks good and progesterone levels are correct, whilst explaining that the chances are slightly lower. This is a shared decision, which also depends on the number of embryos available.

In the most severe cases — a severely damaged uterus, repeated failures despite everything — surrogacy is sometimes considered. In Greece, it is strictly regulated; I explain the conditions in this article.

Organisation for patients who travel

Endometrial preparation can be monitored from home, with ultrasound scans sent to the clinic. You only come to Greece once the thickness is satisfactory. This is one of the reasons why frozen embryo transfer lends itself well to a short stay.

Frequently asked questions