For decades it was thought the womb provided a sterile environment in which an unborn baby was protected from germs until developed its own immune system. A baby's first exposure to bacteria was believed to come from the mother's vagina during birth and followed by skin to skin contact.
But more recently, scientists have been able to detect small amounts of bacteria in the amniotic fluid, the placenta and even in intestines of unborn babies
These findings support idea that the baby's microbiome, or the world of invisible microbes that live inside the body, actually gets established in the womb.
Now a new series of papers, The Microbiome and Childhood Diseases, published this week in the journal Birth Defects Research examines the importance of the microbiome when it comes to the future health of unborn babies.
"The microbiome has become such a hot topic because it represents a crucial population of microorganisms in and on our bodies that is critical to our health. If disrupted, they could cause a wide range of diseases," deputy editor of the journal Dr Michiko Watanabe said.
"The microbiome is not only crucial for adult health, but also for children. What happens during birth and our early childhood could have a big impact on health outcomes through our entire lifespan."
One of the studies reviewed looked at where the bacteria found inside the womb might come from. It found the bacteria might be from the mother's mouth and travel through the blood stream to reach the developing baby through the placenta. Other microbes could travel from the mother's vagina into the womb.
"It indicates that in some infants, seeding of the gut microbiome begins in utero and can be influenced by the health and diet of the mother during pregnancy," the study's lead author Anita Kozyrskyj, from the University of Alberta, told TIME.
"This means that not only do we have to worry about the microbiome of the child but also that of the mother and the irony is that some of our modern medical practices, through their effect on these early microbiota, could interfere with healthy child development."
Despite the growing number of studies focusing on the microbiome of unborn babies, it is too early for researchers to determine what the "right" microbiome for a healthy baby is or what microbiome might result in an unwell child.
"We want to be careful not to be prescriptive and claim that there is a right way to do this and a wrong way," one study author Dr Sharon Meropol told TIME.
"But yes, there are certainly things like maternal dental hygiene and not taking unnecessary antibiotics that pregnant women should consider. We knew it was important for mum to be healthy but now there is even more evidence for how important the maternal environment is.
"Disturbed microbiota could lead to a wide range of childhood diseases including allergies, asthma, obesity, and autism-like neurodevelopmental conditions.
"But what we're learning is that traditional practices like vaginal births, skin-to-skin contact immediately after birth and breastfeeding promote the development of the microbiome in the infant and help set the trajectory towards a healthy life."