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If the woman is a niddah at the time of the wedding, there is a prohibition of yichud between the husband and the wife.
- Gemara Ketubot 4a. The Raavad (Baalei Hanefesh Shaar Haperisha p. 20-22) understands this to be a unique prohibition prior to the couple having tashmish since he has a greater yetzer hara at that time. Therefore, they need a double protection to avoid yichud but there is only a prohibition during the night and not the day. The Baal Hameor argues that the prohibition is the same as regular yichud and there's no room to distinguish between night and day as the yetzer hara doesn't take breaks during the day.