Is a woman permitted to attend shul while on her cycle?
The custom is that she may go to shul while she is having her period with the exception that she does not look at the sefer torah when they do hagba’ah, (when they lift it up to show everyone). Additionally, this only applies to the days that she is actually bleeding, and not for her clean days, even though she hasn’t yet gone to the mikvah. The same halacha would apply to a woman after birth.
Have a happy Chanukah
M:B 88-7, Shiurei Shevet Halevi 195-17 (3), Yesod Hatiharah (Foundation of Purity) 2-20, Halchos of Niddah ( R’ Eider) V-G-2.