There are certain times when a husband and wife may not engage in marital relations even though the wife is not a Nidda, because of the likelihood of her becoming a Nidda at those times. Assuming a woman does not experience bleeding according a precise, consistent cycle, she and her husband must observe three periods of separation each month. The first is at the conclusion of her most recent interval. For example, if her previous menstrual flow had begun twenty-eight days after the onset of her prior flow, then she and her husband must separate at the conclusion of twenty-eight days since her previous flow had begun. They must separate either during the entire day or during the entire night, depending on during which period her previous flow had begun. They must likewise separate on the thirtieth day since the onset of the previous flow, regardless of the duration of the previous interval. Finally, they must separate when one calendar month has passed since the onset of the previous flow. If, for example, her previous flow began on the seventeenth of Marheshvan, then the couple must separate during the day or night of the seventeenth of Kislev.
These periods of separation are called in Halacha "Ona Ha’smucha Le’vestah" ("the period near her cycle").
If one of these periods passes without the sighting of blood, the woman may not resume relations with her husband until she performs an inspection to ensure that bleeding has not begun. This applies to each of the three Onot – at the conclusion of her last interval, the thirtieth day, and the day of the month. On all three occasions, the woman must perform an inspection before resuming marital relations.
Hacham Ovadia Yosef writes (listen to audio recording for precise citation) that a woman should not bathe during any of the three Onot. This includes a bath, swimming pool or other body of water. Since she is prone to experiencing the onset of menstruation during these periods, she should preferably avoid bathing because she might begin bleeding but not see the blood. Nevertheless, Hacham Ovadia adds, a woman who did bathe during her Ona is still considered Tehora ("pure"), and may engage in relations after the Ona, once she performs the required inspection.
Summary: If a woman does not experience bleeding during her "Ona Ha’smucha Le’vestah" (one of three periods when she and her husband must separate due to the likelihood of her becoming a Nidda), she and her husband may resume marital relations, but only after the woman performs an inspection to ensure that bleeding had not begun. During the Ona, a woman should preferably abstain from bathing or swimming.
