Classic formulations of combination hormonal contraceptives create a 28-day cycle. Although convenient for medical calculations, this particular interval is simply a matter of convention. Therefore, a woman may consult with her physician about the possibility of lengthening her cycle by taking the active pills (or extending the ring or patch) for more than 21 days.
The likelihood of breakthrough bleeding increases the longer one continues to take hormones without interruption. It is best to gradually extend the weeks of active hormones over a few cycles until one finds an arrangement that works well.
Some newer formulations of combination hormonal contraceptives are designed to allow the uterine lining to shed only four times a year, or eliminate menstruation altogether. In principle, this creates no halachic problems. In practice, there is anecdotal evidence that these formulations cause significant breakthough bleeding that could make women niddah. Therefore, it may be more practical to extend one’s cycle by combining packs of the standard 28-day pills, with a physician’s approval and supervision.
Extending or manipulating cycles is fairly straightforward with monophasic pills, where all 21 active pills in a package have the same formulation. It is more complicated with triphasic pills, where the dosages of estrogen and progesterone vary over the course of each cycle.
Women who are already taking hormonal contraceptives may be able to adjust the timing to avoid being niddah while traveling or on vacation. A woman may also be able to plan her cycle so that she can use the mikveh before a planned surgery and then remain tehorah for an extended time while she recovers. Although some manipulation is possible, the hormones do need be interrupted periodically to allow for shedding of the uterine lining, and any alteration in cycle length should be discussed with a physician.
Hormonal manipulation for brides seeking to avoid being niddah at the wedding is discussed in the article Setting the Wedding Date.