When a woman is pregnant, her doctor determines a "due date" when the child is expected to be born.
The word "due" is usually used for homework assignments, bill payments, and so on:
When is this project due?
But when you talk about a person being due, it always means the day when a baby is expected to be born:
When are you due?
You can also ask about the child:
When is it due?
You can even ask the father "When are you due?" although of course this question really means "When is (the mother/child) due?"