Implantation is the process by which a transferred embryo attaches to and embeds in the uterine lining (endometrium). For a day 5 embryo, this usually occurs within the first 24 hours following transfer, while for a day 3 embryo it typically happens within 2–4 days.
Without successful implantation, pregnancy cannot develop.
If implantation fails on three or more occasions despite transfer of good-quality embryos, this is known as recurrent implantation failure.
Amongst the various causes for recurrent failures, factors affecting the endometrium should be considered:
Thin endometrium
A lining of <7mm seen on ultrasound during cycle monitoring can indicate possible poor endometrial receptivity with an associated reduced chance of pregnancy, although a recent study found that natural IVF cycles may not be affected in the same way as medicated cycles.
Anatomical issues
There may be fibroids, polyps or a septum disrupting the endometrium and affecting implantation.
Endometriosis and adenomyosis
Both these conditions can be linked to progesterone resistance, and also to suboptimal performance of the endometrium, creating an altered environment which impairs the hormone’s role in maintaining a receptive endometrium.
Displaced window of implantation
In a medicated IVF cycle there is a short ‘window’ when the endometrium is optimally receptive to an embryo – an asynchrony between this window and embryo transfer timing could be a cause of implantation failure.
Endometritis
This is a chronic, low-grade inflammation of the endometrium, which may hinder successful implantation by interfering with embryo–endometrial communication.
When implantation repeatedly fails, taking a full medical history and investigating potential endometrial factors can provide valuable insights and may identify treatable conditions that improve the likelihood of a successful outcome.
However, it is only one aspect of a comprehensive evaluation of recurrent implantation failure and excluding these factors may help identify other underlying causes.
