During pregnancy, a fetus depends on the placenta for oxygen, food and waste disposal, as well as, antibodies from the mother’s immune system. Yet a new study has found that the placenta has no bacteria, leading experts to conclude that it is in fact the mother that provides the microbes that determine a child’s capacity to resist diseases later in life.

In 2014, a study revealed that the placenta contained some bacteria found in the mouth, which suggested that some microbes are passed from the mother’s mouth to her blood and onto the fetus. However, the placenta itself has no bacteria. The latest study, which examined the placentas of more than 500 women, confirmed this fact.

“It’s just about as clear as you could ever ask for,” said Frederic Bushman, a microbiologist at the University of Pennsylvania, who added that researchers are “very skillful with the detective work they did of tracking down multiple sources of contamination.”

In addition, researchers found that some microbes are more predominant when women give birth naturally rather than by cesarean section, which indicates that those bacteria attach to the placenta during delivery. In depth analysis found that the only bacterium present within the placenta itself was streptococcus agalactiae, which was found in just 5% of the samples. This bacterium, however, is a sign of an infection that may cause fatal sepsis in newborns.

Microbiome research initially centered on concentrated microbial communities in the human gut, which houses trillions of bacteria. Yet when researchers examined smaller ecosystems it became harder to differentiate between native colonies and contamination. “I really appreciate the effort made by the researchers to use the best available tools that we have today in order to address this very fundamental question,” said Eran Elinav, an immunologist and microbiome specialist at the Weizmann Institute of Science in Rehovot, Israel.

Still, not all experts are convinced by the latest findings. Kjersti Aagaard, a maternal fetal medicine specialist at Baylor College of Medicine and Texas Children’s Hospital in Houston, who carried out the 2014 study, believes that many of the signals classified as “contamination” are in fact proof of the presence of the placental microbiome. In addition, Aagaard contends that placental microbes prevent the colonization of the fetus by harmful bacteria.

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Meanwhile Nicola Segata, a computational biologist from the University of Trento in Italy, believes the latest study is credible yet says it is still difficult to ascertain that a mother’s womb is a completely sterile environment since it is both unethical and technically challenging to test the fetus’ gut to confirm these conclusions.