Talmudic scholars extended the period of separation so that it lasts a minimum of 12 days. Separation begins at the first sign of blood. According to Jewish Law, the minimum period of menstrual flow is five days. Following the cessation of bleeding, the woman begins counting seven "clean days".
In addition, the rabbis broadened the Law's definition of separation. Whereas the Bible prohibits intercourse, the rabbis decided that a man should not share a bed, or even touch his wife, while she is ritually impure.
The term Niddah refers to a woman who is ritually impure. She is considered ritually impure while she is menstruating and for the seven post-menstrual "clean days", until she has immersed in a ritual bath, known as a Mikvah in Hebrew. According to the Laws of Family Purity, as soon as possible after nightfall of the seventh post-menstrual "clean" day, the woman must immerse herself in a Mikvah. After she has purified herself in a ritual bath, she can resume sexual relations with her husband.
A Mikvah may be either stationary rain water or flowing well/spring water. Oceans, lakes, ponds and springs are all natural catch basins of rainwater, and thus can be used as a Mikvah. This body of water must contain at least 480 liters of water that has not been drawn or stored in a vessel (Lev: 11:36). These days, most Mikvahs exist in cities, and use 480 liters of undrawn water (channeled rainwater or melted snow), and then add water pumped from a faucet to reach a depth comfortable for immersion.
Mikvahs are primarily used by menstrual women and converts to Judaism. Some Jewish men will immerse themselves in the Mikvah before the Sabbath or a holiday. Others use the Mikvah to symbolize a spiritual transition in their lives. The prevalence of the Mikvah in Jewish communities all over the world demonstrates the importance of the Laws of Family Purity to the Jews, throughout their history. Today, however, many Jews do not observe or understand Judaism's Laws of Family Purity.
Most of the Jews who know about and observe a period of sexual abstention do so because it is a commandment from the Torah. There are others, however, who practice Family Purity because they see both physical and psychological benefits from observing a period of separation. Some of these possible benefits include heightened arousal at the time a woman is most fertile and likely to conceive, lowering the chances of vaginal infections and even cervical cancer by not having intercourse during menstruation, and the belief that following these Laws helps a couple form a strong non-physical attachment to each other.