These are one of the funniest kind of jokes because the Medical profession is otherwise a serious profession.
A doctor had just delivered twins. They were a boy and a girl.

The head nurse brought them out for their father to see.

He could hardly believe his good fortune. The girl baby had a pink blanket wrapped around her and the boy baby was enclosed in a blue blanket.

He took one step forward just so he could touch the babies and believe they had finally arrived.

As he started to touch them the nurse took a step backwards and said, "You can't touch those babies. You aren't sterile!"

With out missing a beat, he retorted "You're telling ME I'm not sterile?!"