The Torah instructs: "And you shall not approach a woman to uncover her nakedness [meaning: to have sexual intercourse with her] during her period of [ritual] uncleanliness" (Leviticus 18:19). Since a menstruating woman is considered ritually impure (Leviticus 15:19) because of the loss of potential life, this Torah law forbids normal marital sexual relations during the time of the impurity.

When the monthly menstrual flow begins, a woman becomes niddah (literally "removed" or "separate"). She begins a period of sexual separation from her husband that lasts for the putative five days of the menstrual period, plus another seven "clean" or bloodless days following, for a total of 12 days.

During these 12 days, husbands and wives who observe this law not only do not have sexual intercourse, but also do not kiss or even touch each other. Since affectionate touching can be the prelude to sexual intercourse, this more stringent observance aids in maintaining the sexual abstinence.

The most observant couples have separate beds and during the 12 days will not even allow casual touching. For example, instead of someone saying, "Please pass the salt" and the couple passing the vessel across the table from hand to hand, one partner will set the salt shaker down and the other will pick it up. In this way, nothing that might lead to passionate sexual expression is allowed.

This rule that forbids touching a niddah, a menstruating woman, has led to the custom-with-the-force-of-law under which many Orthodox men do not touch their wives in public — ostensibly protecting her privacy and modesty by not indicating whether or not she is currently a niddah. Nor do these men shake hands with any woman, as they cannot tell whether or not she is a niddah.

This is also why many religious men and women do not dance together in public, or, if they do, will grasp a handkerchief between them rather than hold hands.

On the evening of the completion of the twelfth day (assuming there has been no blood during any of the seven days following the first five days; if there is, the seven days must begin the time of the last bleeding), the woman immerses herself in the mikveh (ritual bath) as an act of symbolic ritual purification.

This is the most prominent and important use of the mikveh, and the reason why every Jewish community must have one.

Following the immersion, normal marital sexual relations are resumed.

These laws of sexual abstinence, followed by ritual purification in a mikveh, are called taharat hamishpacha (laws of family purity), for they are intended to elevate the relationship between husband and wife to a level of sacred holiness.

The Torah makes clear that sexual abstinence with a menstruating woman has nothing to do with physical cleanliness or hygiene. Menstruation is not "dirty," and a menstruating woman is not actually "unclean."

As with all other instances of ritual impurity, the reason a menstruating woman is considered ritually impure — ineligible to participate in Jewish ritual observances (in ancient days, the bringing of sacrifices to the sanctuary) — is because she is experiencing the loss of both body fluids and, most importantly, potential life.

So this "uncleanness" or "impurity" is not at all physical, but wholly ritual.

Husbands and wives who observe the laws of taharat hamishpacha affirm that the sexual relationship between them need not be left unregulated, where it has the potential of being reduced to the most basic animalistic needs and demands. In this most intimate of all human relationships, there can and must be care, consideration, respect and, above all, self-control. Taharat hamishpacha proves that human beings can in fact control their passions and restrain their instincts.

Couples who observe these laws also report that their marriages retain a "freshness." Each partner maintains his or her own monthly "space," a time of privacy. And together, the couple renews the sexual relationship each month with an eagerness and a passion reminiscent of their honeymoon.

Taharat hamishpacha is Jewish birth control — in reverse.

In a normal menstrual cycle, a woman is able to conceive in the middle of the cycle — the 13th, 14th and 15th days of the 28-day cycle. It is virtually certain that after 12 days of sexual abstinence, such a couple will have intercourse on the very days when the woman is most likely to conceive.

In a time of sexual freedom, taharat hamishpacha may seem quaint and outmoded. Yet not only Orthodox couples, but also a growing number of young couples from the more liberal denominations of contemporary Judaism are embracing the laws and enhancing their lives this way.