It is forbidden for a husband and wife to have sexual relations if the wife has had even a droplet of menstrual bleeding (183:1); such bleeding makes her a NIDDAH. When a woman becomes a NIDDAH she must wait until sunset of the day the bleeding stops or the fifth day after it started, whichever is later; see 196:11,13. During that day, preferably close to sunset, she must carefully examine herself internally, and should keep a tightly fitting white tampon inserted until after dark, to confirm (by the absence of blood on the tampon) that her bleeding has stopped (196:1-2). She must wash herself, and wear clean white underclothing and sleep on clean white sheets for seven consecutive days (196:3; see 196:12). On each of these days she should examine herself internally twice, once in the morning and once at twilight — or at the very least once on the first day and once on the seventh day (196:4-6). If she finds blood in any of these examinations, or finds a bloodstain on her clothing or bedding during the first three days, she must once again verify that her bleeding has stopped and restart the count of seven clean days (196:10). After completing the count she must immerse herself (see Ch.15); when she has done this she is no longer a NIDDAH and may resume sexual relations (197:1). She should preferably immerse herself on the night after the seventh clean day, or on a subsequent night if her husband was absent earlier (see 197:2). If immersion at night is impractical, she may immerse herself in the daytime of the eighth (or subsequent) day. There is a dispute regarding if in extreme circumstances she may immerse on the seventh day before nightfall as well (see 197:3-5).
Any reddish or blackish substance is regarded as blood (see 188:1,4-6) provided it comes out of the womb normally (see 188:3); on trustworthiness in these matters see 188:2. If a woman who has an internal lesion finds blood she may usually assume that the blood comes from the lesion rather than from menstruation (see 187:5-7,13;188:3,5). On a woman who finds blood after urinating see 191:1.
If a woman’s menstrual bleeding begins at regular intervals or under particular conditions or in association with particular symptoms, once this has happened three times she must assume that the pattern (“period”) will continue, and on each such subsequent occasion she must abstain from sexual relations for 12 hours and examine herself, until she has found no blood on three such occasions (184:1-2,4-6,8-9,11-12). If she has not established a regular pattern she must only assume that she will bleed on the first repetition of the conditions under which she last bled or on the thirtieth day after she last bled; at these times she must also abstain from relations for 12 hours and examine herself (189:1-26,32). At other times a woman who has a regular period may have relations without examining herself; but if she does not have a regular period, according to many authorities she must always examine herself before she has relations and both she and her husband must examine themselves afterwards (186:1-4). If a woman finds blood on three successive occasions immediately after having relations she becomes forbidden to her husband unless it can be established that the blood is not from the womb or unless her condition can be cured (see 186:5;187:1-4,8-12); when relations are impossible the couple should divorce (see 187:14). On special rules for women at the age of menarche or menopause or pregnant or nursing women see 184:3,7; 189:27-31,33-34.
Shulchan Aruch, Copyright (c) 2000 ProjectGenesis, Inc.