How do foetuses/babies live under water for 9 months?

Foetuses/Babies live for 9 months in amniotic fluid in the placenta and in this time they even learn to breathe in and out the fluid. How do they breathe under the water and then when they're born the breathe oxygen? And also, if we live under water for 9 months of our lives, how come we struggle to breathe under water now?