If unknown causes result in several miscarriages, or implantation cannot be detected with several embryos, it may be advisable to examine the lining of the uterus (the endometrium) to clarify any issues. However, it is important to know that these test procedures are experimental, particularly as the efficacy of subsequent treatment has not been proven with sufficient studies.
The thickening of the endometrium is influenced by various - above all hormonal - factors throughout the menstrual cycle, leading to structural and functional changes. In the different phases, cell function and the production of cell components and proteins varies. The ‘blueprints’ for this are found in the genes, which have different activities depending on the point in the cycle. A certain gene expression profile is typical, or ‘correct’, at the time of embryo implantation, marking the implantation window, which is generally 6 days after ovulation.
In rare cases, this implantation window can be shifted forwards (e.g. 3 days after ovulation) or backwards. This can be determined by examining the endometrium. For this, a small sample is taken at the usual time in the cycle, and genetic testing (ERA test) is carried out. If the implantation window is offset, the embryo transfer time must be adjusted.
Usually, when a foreign organism intrudes, this causes an immune response. But this is not the case when an embryo implants in the endometrium: this is selectively accepted. Specific immunological mechanisms in the endometrium that have not yet been fully explored are responsible for this.
Particular attention has been paid to the ‘natural killer cells’ - a population of white blood cells, which have special tasks here and are apparently required in a specific composition to control the implantation process. In some women, this composition seems to be impaired. This can be diagnosed using endometrial biopsies. The treatment for such issues has not yet been established, and should therefore take place in the context of clinical studies.
In rare cases, women may be diagnosed with chronic inflammation of the uterine lining, or endometritis, particularly where specific immune cells (plasma cells) are detected. This is done using endometrial biopsy. If chronic inflammation is found, antibiotic therapy over several weeks can be attempted. This is also an experimental treatment.