The 346th prohibition is that one is forbidden from having relations with a woman when she is still menstrualy impure, i.e., for seven days.1
The source of this commandment is G‑d's statement,2 "do not approach a woman who is menstrually impure." She remains in this state even longer than seven days as long as she does not immerse [in a mikvah].3
One who transgresses this prohibition intentionally receives kares; and if it was unintentional, must bring a sin-offering.