This section is written in partnership with UR Vistahermosa experts from Spain.

Endometrial microbiota

The importance of its study in assisted reproduction

For many years it was thought that the endometrium (the inner layer of the uterus, where the implantation of the embryo occurs) was a sterile organ. However, several studies showed that different groups of microorganisms (mainly bacteria and fungi) live in the endometrium.

Together, this is known as the endometrial microbiota. The presence of these microorganisms is closely linked to our health. Thus, when there is an imbalance in these colonies, various diseases usually appear.

It has been observed that the microbiota may have an effect on the course of the pregnancy during its various stages:

  • During the implantation of the embryo into the endometrium. It may alter the receptivity of the endometrium and condition its preparation so that the embryo will adhere.
  • During pregnancy. Its alteration may be one of the causes of premature births and miscarriages.

This is why the subject of the microbiota has become more relevant in the field of gynaecology and assisted reproduction. Knowledge of its implications for fertility may help improve the outcome of our treatments, including in patients for whom the cause of sterility or infertility is currently unknown. Ifwe delve into the study of the urogenital microbiota, we see that what is normal and desirable in healthy women is that most of the bacteria in the reproductive tract belong to the Lactobacillus genus, although other genuses – such as Gardnerella, Atopobium, Prevotella and Acidobacteria may also be found.

Another important fact is that approximately 20% to 30% of women of reproductive age have altered vaginal flora. That is, it is not dominated by lactobacilli, but rather has another type of flora that we may consider pathogenic or dysbiotic. For women involved in assisted reproduction processes due to a fertility problem, this prevalence rises to 40%.

How does the endometrial microbiota influence fertility?

  • Direct cause. Some pathogenic bacteria are a direct cause of infertility. For example: bacteria that cause gonorrhea or trachomatis; some species of Mycoplasma or Mycobacterium tuberculosis, which cause genital tuberculosis.
  • Gametogenesis. Sperm quality may be altered, depending on which bacteria predominate in the semen sample. It has also been discovered that areas that were believed to be sterile, such as the ovarian follicles, have a very active microbiota.

Endometrial receptivity and pregnancy

The endometrial microbiota may influence its ability to allow an embryo to be implanted and condition success when attempting to become pregnant. It may also influence more advanced stages of pregnancy. For example, a premature birth, where the premature rupture of the membrane occurs or even when it ends in a miscarriage.

Recent studies show that women with 90% of the flora dominated by lactobacilli in the uterine cavity have better rates of implantation, gestation and live birth compared to the group of women with a receptive endometrium with a lactobacilli population of less than 90%.

Thus, the low presence of lactobacilli in the uterus is related to a worse reproductive prognosis, and may be the of some implantation failures and miscarriages.

Strategies to improve the microbiota and increase pregnancy rates

We have different techniques for studying the type of microorganisms that colonise our genital tract. For example, vaginal and endometrial cultures to detect asymptomatic infections or chronic endometritis.

Currently, research is being undertaken on the benefit of modulating the microbiota to improve the results in assisted reproduction techniques by administering biotherapeutic products containing one or more specific bacterial trains (lactobacilli) that are administered to colonise the relevant niche, while simultaneously displacing the dysbiotic bacteria.

This is why orally administered probiotics have been added as adjuvants to the assisted reproduction treatment. This treatment is well tolerated and makes it possible to effective colonise both the vaginal mucosa and the endometrium. Studies indicate that the administration of lactobacilli should be initiated from at least the beginning of ovarian stimulation with preparations containing, among other subtypes, the Lactobacillus rhamnosus. Given that probiotics beneficial effect on the prevention of premature abortions and births is known, treatment may be prolonged during pregnancy.


The microbiota is another key part of the complex mechanism of human reproduction. New knowledge highlights the need to act on it in couples who come in with reproductive problems, especially with a history of implantation failures. Since there is a correlation between the vaginal and the endometrial microbiotas, the therapeutic strategies we have seem to be useful in improving the reproducive prognosis of these couples.

Article source:
This article has been published in Creando Familias magazine by UR Vistahermosa clinic from Spain.

Picture of Dr. Susanna Malkhasian
Dr. Susanna Malkhasian
Dr. Susanna Malkhasian is a renowned specialist in Gynaecology and Obstetrics. She currently works as a gynaecologist at UR Vistahermosa and is an expert in Assisted Reproduction, risk pregnancy and laparoscopy. She studied Medicine and Surgery at the Miguel Hernández University of Elche and later specialised via MIR at the Marina Baixa Hospital. She also completed a Master's Degree in Human Reproduction at the Universidad Rey Juan Carlos.

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