Your sister announced that she's pregnant. You congratulated her, and now you want to find out more information about it. You ask:
When are you due?
When someone is pregnant, their 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 you talk about a person being due, it always means the day when she is expected to give birth.
You use the phrase to talk about the mother ("When are you due?") but you can also use it to talk about the child:
When is it due?
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