Actually the rules on children are a bit more detailed than only being allowed one child.
Farming families are allowed to try a second time after having a girl because the law recognises the importance of having boys to help with the labour.
I have a friend on the internet who lives in china so I asked her about this once and she was talking about it in far more detail, but its certainly not as simple as that.
Also, the chinese class their birthday as the day of conception
