“When are you due?”
Your sister announced that she's pregnant. You congratulated her, and now you want to find out more information about it, so you ask her this.
When are you due?
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When (is someone) due?
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?"