“family purity;” laws regarding Sexual Discipline in Marriage. In marital law, immersion of the wife in a Mikvah is a vital component. These laws are based on the verse: “You shall not come near a woman, while she is impure by her uncleanness, to uncover her nakedness” (Lev. 18:19).
Quoting several paragraphs from “To Be a Jew,” by Rabbi Chaim HaLevy Donin, published by Basic Books, N.Y., 1972:
“For a full seven day period from the onset of the monthly menstrual period, the Torah prohibits all sexual relations between husband and wife. The technical term for the state in which the wife is in during the menstrual period is called “niddah” (literal meaning: to be removed or separated).”
“By Rabbinic edict, the Talmud extended this period of separation (niddah) to “seven clean days” following the menstrual period. Since the menstrual period lasts about five days for the average woman, the total period of separation that is in force each month is about twelve days.”
“A woman remains in the state of ‘niddah’ until she has immersed herself in a ritual body of water [‘mikvah’]. This immersion is the ritual act that divides the two periods of time – the period of separation when marital relations are forbidden and the period of union when such relations are not only permissible but regarded as essential to physical and mental health.”
“It is the responsibility of the wife to note carefully the day on which she last saw blood and to count the seven “clean” days that follow, so that her visit to the mikvah is neither too soon nor unnecessarily delayed.”