This is because vaginal birth exposes the child to maternal bacteria. This is harmless as the child still carries the mother's antibodies. However, this does do wonders for getting the child's immune system going and results in fewer immunological issues later in life. C-sections are a great alternative to "died in childbirth", but women who choose them for convenience are crippling their childrens' immune system for life. As in, all their children. Because once you have one, you're not giving birth naturally.