Bacteria do not colonize babies until the moment of birth, but the mother’s microbiome can still influence fetuses in key ways, say McMaster researchers Kate Kennedy and Deb Sloboda.

Previous claims that humans are colonized by bacteria before birth would, if true, have major implications for clinical practice and undermine established principles in immunology and reproductive biology.

Sloboda and Kennedy are co-authors of a study published in the journal Nature earlier this year.

“The idea that babies are colonized in utero, before birth, contradicts our understanding of fetal development and immune education,” said Kennedy, first study author and a postdoctoral researcher in the departments of obstetrics and gynecology, and biochemistry and biomedical sciences, and the Farncombe Family Digestive Health Research Institute.

The 46 leading experts who contributed to this study unanimously concluded that babies are not colonized before birth, and that erroneous findings of microbes in fetal tissues were due to contamination of samples during vaginal delivery, clinical procedures, or during laboratory analysis.

“We know that babies are initially colonized by microbes during and after birth, not before,” Kennedy said.

“But this leaves a lot of important questions left to answer. How do babies prepare for this sudden exposure to our microbial world? How does the mother’s microbiome shape prenatal development? What factors influence which microbes will be passed on from the mother to her baby?”

While fetuses in the womb do not harbour live bacteria, Kennedy said research should focus on the maternal microbiome during pregnancy. Molecules produced by the mother’s microbes, known as metabolites, cross the placenta even though gut bacteria themselves do not.

“What we need to do as researchers is focus on avenues of investigation that are supported by evidence,” said Deborah Sloboda, professor in the Department of Biochemistry and Biomedical Sciences.

There is a “big gap in the literature,” in understanding how the maternal gut changes during pregnancy and more investigation is needed into how the microbiome influences maternal metabolic adaptation and the intra-uterine environment, Sloboda said.

“There is growing evidence that the maternal gut microbiome likely plays an indirect role in the physiological adaptations of pregnancy, through a number of signalling pathways,” said Sloboda.

While the maternal microbiome plays a key role in infant development, the makeup of an individual’s microbiome is determined by a number of factors, such as where they live, and even elderly people will still have some gut bacteria in common with their mothers, she said.

“This paper is important because it can focus our investigative efforts on things that will have real impact on maternal and child health,” said Sloboda.

“Rather than spending precious resources and time trying to solve a controversy, we should be looking into all the unanswered questions around pregnancy and how our early life environment impacts lifelong health.”