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?"