Researchers in Germany have found evidence that baby mice squeezing through the birth canal swallow bacterial molecules that give their immune systems a head start over those born through a caesarean section. Mice born through the vagina experienced an inflammatory response in their gut within two hours after birth, a sign that their cells had been stimulated by bacterial molecules. Mice born by caesarean, however, did not show signs of such activation until being fed fragments of bacteria after their birth.

If the pattern is followed in humans, this information could add to the persistent and vigorous debate about whether caesareans carry greater risks for a mother and baby than vaginal birth.