Babies heads are born with 4 seperate bones which mould as needed. Also your pelvis swings like gates to open and accommodate. The vagina is also like an accordion, and stretches like a rubber band and bounds back into shape.... here's a really obvious example of how things work - when you poo!!! It stretches and then goes back to shape to pass waste, thats its job just like the vagina but it's job is to pass a baby through. It's made for it!