The success rates of Assisted Reproduction treatments are increasingly higher, but, unfortunately, they are not at 100%. This means that, in a very positive way, new avenues of research are opening up in order to improve this result.

Microbiota is currently a very trendy topic, just as immunology was five years ago. However, these are studies which, to date, do not have definite scientific evidence and, therefore, are not proposed as a basic fertility test before starting treatment, but rather as a way to explore in those patients who have previously had a miscarriage after AR treatment and are always aiming to improve their probability of pregnancy.

What is the microbiota?

  • MICROBIOTA: Set of microbes found in our organism.
  • MICROBIOME: Set of genes and their metabolites together with the host (or surrounding environment) which in this case is us.

We are holobionts, that is, beings formed by groups of microorganisms within our organism forming an ecological unit. In our human body there are 10 times more bacteria or microbes than human cells.

This collaborative relationship between these microbes and human cells is so important that it has been suggested that the set of genetic information carried by these microbes can be considered the second human genome. However, with regard to the extent to which it influences fertility, what we really have to consider here is the role of the microbiota in the reproductive process.

Is alteration of the microbiota a cause or a consequence of miscarriage?

Dr. De Prados explains that on many occasions this alteration of the microbiota refers to the existence of a previous pathology that is producing an inflammation and consequently allows pathogenic bacteria to grow that can affect the results of assisted reproduction treatments.

Some very common examples in AR are endometriosis, an inflammatory disease that affects 15% of women of childbearing age, in which this generalized inflammatory state can lead to an alteration of the microbiota at the endometrial, vaginal and intestinal levels. In this sense, the alteration of the microbiota is a consequence of the disease.

The same occurs with hydrosalpinx (the accumulation of fluid in the tubes). In these cases, a growth of pathogenic bacteria will be produced, generating a dysbiosis and, therefore, a possible very negative impact on the embryo implantation process and even on the evolution of the pregnancy. And so with myomas, polyps, adenomyosis or even celiac disease.

Which kind of patients should have their microbiota studied?

About five years ago, the lines of research were focused on immunology, and today they have evolved towards the study of the microbiota and the microbiome. However, these lines of research make us place ourselves in studies that today do not have proven evidence in any type of patient, which is why the study of the microbiota is not a routine fertility test at the beginning of a treatment, but rather it is proposed in the event of failure, that is, in those who after undergoing treatment have not achieved pregnancy.

  • Patients with miscarriage.
  • Patients with implantation failure.

At present, microbiota studies are not performed for men, since what is really being considered is the interaction between the endometrium and the embryo in order to study whether the bacterial composition of the endometrium affects the implantation and subsequent evolution of the embryo.

Symptoms and procedure

An alteration in the microbiota may be asymptomatic, but it may nevertheless present:

  • Vaginal discharge
  • Irregular bleeding
  • Itching
  • Mild pain

The main diagnostic test in the classical way is through a culture. But it has its limitations. Ideally, a hysteroscopy-guided biopsy should be performed to see susceptible areas.

After a few days we get a full report of the uterine microbiome, as well as the presence or absence of pathogenic bacteria.


As of today we cannot affirm what a physiological endometrial microbiota consists of. We could define it as that composed of a set of microbes that is tolerant with the implantation and evolution of the embryo, and not with the existence or absence of a specific bacterium.

About Dr. Adolfo de Prados

Dr. de Prados graduated in Medicine, specializing in Gynecology and Obstetrics at the University of Valencia, and subsequently completed a Master’s Degree in Assisted Human Reproduction at the same university.

He has an extensive career in the field of Reproductive Medicine, as well as in the field of teaching as an adjunct university professor at prestigious universities in the Valencian Community.

Passionate about the microbiota, he has been a training speaker on this subject at the Congress of Valencia organized by the General Hospital and provides specialized training to professionals in the health sector.