The Way You’re Born Can Mess With The Microbes You Need To Survive

Throughout the animal kingdom, mothers transfer microbes to their young while giving birth. Different species of tadpoles acquire specific skin bacteria from mother frogs even though they all live in the same pond with the same bacterial background. Emerging chicken eggs get inoculated with microbes from a bacteria-filled pouch near the mother hen’s rectum. And for millennia, mammalian babies have acquired founding populations of microbes by passing through their mothers’ vagina. This microbial handoff is also a critical aspect of infant health in humans. Today it is in peril.

The Symbiotic Relationship Between Microbes and Mothers

Microbes play a hidden role in the course of every pregnancy. During the first trimester, certain species of bacteria become overrepresented while others become less common. By the third trimester, just before the baby is born, even greater shifts occur. These changes, involving scores of species, are not random. The compositions change in the same direction across the dozens of women who have been studied.

Many physiological and pathological features of pregnancy are controlled, at least in part, by the mother’s resident microbes, which evolved to help her and themselves. When food is in short supply during pregnancy, as has often occurred in human history, the mother’s microbes will shift their net metabolism so that more calories flow from food to her body. In this way, her microbes increase the odds that there will be a next generation, one that will provide a new home for them.