Hormonal regulation ("the pill") is usually the birth-control method of choice because its effect is temporary, because it creates no physical barriers to intercourse, and because it is over 99% effective.
The most common type of oral contraceptive contains a combination of estrogen and progesterone. These pills are generally arranged to create a very regular 28-day cycle. A woman takes pills for 21 days – either the same pill, or a series of pills in fluctuating doses of estrogen and progesterone – during which time she is unlikely to bleed. She then stops taking the pills for a week, which causes her uterine lining to shed. Within a few days after she stops taking the pill, she should begin to bleed, at which point she becomes niddah. Combinations of estrogen and progesterone are now also available in the form of patches and vaginal rings. The patch is placed on the skin weekly for three weeks in a row and the ring is left in the vaginal canal for three weeks. At the end of the three weeks, the patch or ring is removed for a week, allowing for a withdrawal bleed to take place.
The 28-day cycle, although convenient for medical calculations, is simply a matter of convention. If a woman wants to generate a longer cycle so that she becomes niddah less frequently, she should discuss with her doctor the possibility of taking the active pills (or extending the ring or patch) for more than 21 days. It may also be possible for a woman already taking oral contraceptives to adjust the timing of the pills to avoid being niddah while traveling or on vacation. Although some manipulation is possible, the hormones must be interrupted periodically to allow for shedding of the uterine lining, and any alteration in timing should be discussed with a doctor.
Some hormonal contraceptives contain only progesterone. Progesterone-only formulations are favored by some physicians for use while breastfeeding because they are reported to have less effect on milk production.
Some women hesitate to use hormonal contraceptives because of health concerns. In fact, the risks from the modern low-dose estrogen preparations are negligible for most women. Some women, however, have a medical condition (e.g., liver disease, high blood pressure, clotting problems) or a family medical history that rules out the use of this method or makes it less desirable. All women should see a physician and undergo an appropriate history and physical exam before beginning oral contraception. A follow-up exam 1-2 months after starting the pill to check blood pressure is also recommended.
Contemporary rabbinic authorities differ about the effect of the artificial regular cycle produced by the hormones on the calculation of vestot (see Vesatot and Hormonal Contraception). Some authorities maintain that a woman should continue to keep her usual onot perishah, regardless of the fact that she is taking the pill. Others rule that she should base her calculations only on the pill, and still others impose the stringencies of both positions. A woman should ask her rabbi how to proceed.