2,090. Patterns of Six and Seven Days

Hilchos Issurei Biah 7:19

If a woman’s pattern is to see blood for six impure days followed by six pure days, she may engage in intimacy on five days after her niddah days. After that, she may never engage in intimacy again.

Hilchos Issurei Biah 7:20

If a woman’s pattern is to see blood for seven impure days followed by seven pure days, she may engage in intimacy during the first pure week after her niddah days. This is followed by an impure week which makes her like a zavah. The week after that is needed to count her seven clean days and she is not permitted to be intimate during that week. Therefore, in four weeks she is only permitted to engage in intimacy for one week. For the rest of her life, she may engage in intimacy for 18 days over 18 weeks, as follows: in the fifth week, she’s a zavah. In the sixth week she’s pure but must count her seven clean days, In the seventh week, she’s a zavah. In the eighth week, she must count. In the ninth week, when she sees blood, five days are niddah days and two are the start of her zivah days. She must therefore watch one day of the tenth week and she may engage in intimacy for six days of that week. In week 11 when she sees blood, two days are the end of her zivah days and five fall during her niddah days. In week 12, which is pure, she’s permitted to engage in intimacy for five days. In week 13, she’s a zavah. In week 14, she must count. In week 15, she’s a zavah. In week 16, she must count. In week 17, she’s a zavah. In week 18, she must count. She must continue to count this way for the rest of her life, with the result that she will only be able to engage in intimacy on 18 days over 18 weeks. If she didn’t have this biological anomaly and would be a niddah for a week followed by 11 pure days, she would be able to engage in intimacy for 11 weeks’ worth of days, i.e., 77 out of the 18 weeks.