Far from being innate, the microbiota is built gradually in the first years of life. It is at the time of childbirth and the first hours of life that the baby's intestine, previously sterile, is colonized by bacteria. Indeed, birth is a crucial stage, since it is the moment when the baby leaves its mother's womb, a protected environment, to find itself facing an environment populated by bacteria. During a vaginal delivery, the baby's passage through the vaginal canal ensures the first bacterial contact with the mother's vaginal flora. Hence the importance of maintaining a healthy and balanced vaginal flora during pregnancy. Unfortunately, this phenomenon does not occur in the case of a cesarean delivery. This inheritance of good bacteria, with probiotic potential, is the foundation of the baby's future intestinal microbiota, which it builds during the first 3 years of its life. Indeed, a constant dialogue is established between the microbiota and the baby's immune system. The latter will then be able to learn to tolerate good bacteria and fight against bad ones.