Placenta and amniotic fluid are sterile: microorganisms colonize humans only after birth

Enlarge image

According to the study, detection of microbiomes in unborn babies is due to contamination of samples

Photo: Guido Mieth / Getty Images

Bacteria and other microorganisms belong to humans and are important for healthy skin and digestion, among other things.

However, scientists have been arguing for years about when they colonize the body.

An international team has now found that the womb is usually sterile and babies only come into contact with microorganisms at birth.

This was a common assumption for a long time, but recent studies have reported that even amniotic fluid and placenta samples contained microorganisms.

A consortium of 46 experts from reproductive biology, microbiome science and immunology vehemently contradicts this more recent thesis of the fetal microbiome.

The study was published in the journal Nature.

Important for the immune system

Our microbiome, i.e. the entirety of all microorganisms that are in and on the skin, mucous membranes and organs, plays a central role in our health and our immune system.

Even newborns already have an immune system – but scientists are still asking questions about its development.

Among other things, they want to know when a fetus first comes into contact with bacteria, viruses and other microbes.

For a long time it was believed that the unborn child and the uterus in which it grows, including the placenta and amniotic fluid, are sterile in a healthy pregnancy.

However, since 2010, several research teams have reported finding bacteria in samples of the placenta and amniotic fluid.

From this, they concluded that a fetal microbiome was present.

Researchers led by Jens Walter from University College Cork have now reviewed the analyzes of the studies discussed from their respective specialist perspectives.

They unanimously concluded that the detection of microbiomes in unborn babies is due to contamination of samples.

Differences in cesarean and vaginal births

For example, amniotic fluid samples differed significantly in caesarean and vaginal births, which should not be the case for a general fetal microbiome.

During a natural birth, the children get a large number of microorganisms on their way through the birth canal.

"We are aware that our position is at odds with dozens of publications providing evidence of microbial populations in the womb, but we believe in the validity of our multi-pronged approach," the authors write.

A fetal microbiome would contradict what is known about human biology, Walter explains.

"For example, we know that the placenta is full of anatomical and immunological barriers that prevent microbes from entering and colonizing it." In fact, very few microorganisms can cross the placenta and then lead to fetal infection, including the herpes virus , which causes cytomegalovirus infection, rubella and chickenpox viruses, and listeria.

The experts hope their consensus will provide guidance for future research.

"It creates a solid scientific basis to concentrate research efforts where they are most effective," says Walter.

Knowing that the fetus is in a sterile environment confirms that bacterial colonization occurs during birth and in the early postnatal period.

It is still important to find out how the fetus's immune system develops, explains Walter.

However, the focus of research should shift away from living microbes towards the cell components of microbes and the chemicals they produce, so-called metabolites.

"Such compounds have been shown to cross the placenta and prime the fetus's immune system for life in a germ-ridden world."

ani/dpa