1 How come you didn't give birth in water?---------多用于口语,尤其是熟人之间.
=== How did it come that you didn't give birth in water?
其中,How did it come that you didn't give birth in water--------是主语从句.
How did it（it是形式主语） come that（that是主语从句连接词） you didn't give birth in water（后面真正主语）
2 Why didn't you give birth in water?----------why是标准的用法,书面的和口语的.