A lot happens during pregnancy. In the months before birth, a baby’s brain forms, their heart thumps for the first time and their fingernails sprout. At some point, a baby’s gut, skin and other organs will be populated with trillions of beneficial bacteria — the microbiome. But does that happen before birth, or after?
For more than a century researchers thought the human uterus — and the placenta within — was sterile. According to the “sterile womb paradigm,” they believed that babies only acquired microbes during and after birth. Over the past few decades, however, a handful of research groups have reported finding small amounts of bacteria in the placenta. In 2014, one team used gene sequencing to identify what they found, leading some scientists to reconsider their stance. Others still disagree, arguing that those bacteria came from contamination. In Science Smackdown, we let experts argue the evidence on when baby meets microbe.
