What is the ruling on intercourse if a husband says to his wife, 'If you become pregnant, you are divorced'?

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He should have intercourse with her once during every period of purity following menstruation. This is because menstruation is a sign that her womb is clear of pregnancy, and intercourse is the cause of pregnancy. If he has intercourse, he should then abstain from her due to the possibility that she became pregnant from that act, in which case she would be divorced by it.