A childhood belief about rude bits

I had always bathed with my brother and was well aware that his bum and mine were different, although for me the biggest difference between us, the one that marked him as male and me as female, was that his hair was short while mine was long. Ok. However I began to wonder how doctors and nurses and what-have-you could tell what babies were boys and which were girls at birth, when both had short hair. Surely they didnt look at their bums?