A woman observes certain days each month when she is considered halachically most likely to begin her menses. These are called veset days or onot perishah (times of separation). On each of these days, the woman performs a bedikah and the couple abstains from intercourse.
A halachic question arises when a woman's menstrual cycle is controlled by hormonal treatments. For example, a woman taking combined estrogen and progesterone (the most commonly prescribed oral contraceptive) will generally begin bleeding about 2-4 days after she stops taking the active pills. The calculation of vestot in such circumstances is a matter of debate among contemporary authorities. There are three schools of thought:
- One approach is to ignore the hormonal contraceptives. The woman keeps the usual vestot (generally the Hebrew date, the interval, and the thirtieth day).
- Another approach is to consider only the effects of the hormones. This veset is based on the interval between the cessation of active pills and the onset of menstruation, rather than on days elapsed since the previous menstrual cycle. The woman initially observes as her veset only the period from two to four days after cessation of active pills, when she is likely to begin bleeding. If, for three consecutive times, she begins bleeding on a specific day after she stops taking the active pills, then she establishes a veset kavua. If, for three consecutive months, she consistently begins bleeding within a limited range of onot (e.g., 6-8 onot after stopping active pills), then she observes an extended veset during that entire range of onot.
- A third approach starts with number 1 and continues with number 2. The woman intially observes the usual vestot. But if, for three consecutive times, she begins bleeding at a consistent interval after she stops taking the active pills, then this alone determines her future vestot.
One should consult one's individual rabbi as to how to proceed in practice. The halachic approaches are also relevant when a woman stops using hormonal contraception, and a rabbi should be consulted at this point as well.
Detailed discussion of this debate can be found in Nishmat HaBayit, simanim 38-41 and the articles by Rabbi Shlomo Levi in Techumin Vol 3 and Rabbi Sinai Adler in Techumin Vol 4. In English, see The Laws of Niddah by Rabbi Binyomin Forst, who addresses the issue on pp. 363-368 of Volume 1. See footnote 38 where he discusses the different approaches.
For women using patch or ring forms of contraception, removal of the patch or ring is equivalent to the cessation of active pills.