A woman anticipates her menses on certain days, based on her previous menstrual pattern. These days are called onot perishah (times of separation) or veset days. Most women observe onot perishah on yom hachodesh, the haflagah, and onah beinonit.
Marital relations are prohibited on an onat perishah in order to minimize the risk that menstruation could begin during intercourse, causing an inadvertent violation of the laws of niddah. Other forms of affection are permitted; each couple should use common sense in deciding what’s appropriate. There is a custom in some communities to refrain from hugging and kissing. Additional restrictions that apply when the wife is a niddah (harchakot) do not apply on an onat perishah.
During each onat perishah, a woman must check whether she has begun to menstruate as anticipated. Because she becomes niddah as soon as blood exits the uterus into the vaginal canal, even before it leaves her body, she may not rely on an external inspection alone. She needs to perform at least one internal examination (bedikah) at some point during the onat perishah. Some women do two bedikot (beginning and end) or three (beginning, middle and end), but if the examinations are painful, or if the irritation could itself lead to a problematic bedikah, then one is sufficient.
If a woman forgot to perform a bedikah on the onah beinonit (the thirtieth day from her previous menstrual period) or on her veset kavua (established period), she is required to do one afterwards and may not resume relations with her husband until she has done so. If, however, she missed the bedikah on another onat perishah, it is desirable but not required that she do one afterwards.
Duration of Onot Perishah
The word onah literally means “time period.” In the context of the laws of niddah, it usually refers to a day or a night. The daytime onah begins at sunrise (henetz hachamah, commonly called netz) and ends at sunset (shekiat hachamah or shekiah). The night-time onah lasts from sunset until sunrise.
Each onat perishah lasts for one onah, either daytime or night-time, corresponding to the beginning of the previous menstrual period. For example, a woman who began menstruating on the 15th of Nisan at night will observe an onat perishah on the 15th of Iyar at night.
Some couples observe the onah prior to the veset as an additional onat perishah. This custom is known as the onah of the Or Zarua (named for the 13th century authority who argued for it). If, for example, a woman began menstruating on the 15th of Nisan at night, the regular onat perishah falls on the 15th of Iyar at night, and the additional onat perishah of the Or Zarua falls on the previous onah, the 14th of Iyar during the day. Hence, a couple observing this practice must refrain from relations during the 24-hour period, from sunrise on the 14th to sunrise on the 15th.
Onot over the Life Cycle
Pregnant, nursing, and postmenopausal women are sometimes presumed to be amenorrheic (without a monthly cycle – in Hebrew mesuleket damim). The laws of onot perishah in these situatuons are discussed in our articles on Pregnancy, Breastfeeding, and Perimenopause & Menopause.