Fig. 1.

Schematic representation of the opposing concepts by which human microbiota is acquired early in life. a In the sterile womb paradigm, the placenta, amniotic fluid, and fetal gut remain sterile during a healthy pregnancy, and the early microbiome is acquired during and after birth. Accordingly, the gut microbiota of infants born vaginally resemble the microbiota of the mother’s vagina, while the microbiota of infants born by cesarean section are similar to the mother’s skin microbiota. b The “in utero colonization hypothesis” proposes that some microbial members of the infants’ gut microbiome are acquired before birth, probably via contact with a placental microbiome, which has been suggested to originate from the mother’s gut or oral microbiome