Abstract: There is no halachic objection to hormonal manipulation of the menstrual cycle. This can be accomplished either with hormonal contraceptives or with progesterone towards the end of the cycle. At times, women will request this intervention for lifestyle considerations. Such assistance should be offered unless medically contraindicated. Efforts should be made to minimize the chances of breakthrough bleeding to allow the woman to exit the niddah status.
Discussion: A woman is required to follow the rules of niddah when she experiences uterine bleeding not due to injury. However, she is not obligated to become niddah on a monthly basis. In fact, regular monthly menstruation is a relatively recent phenomenon. The average woman at the time of the Talmud would have menstruated 160 times in her life, compared to an average of 450 today . Declines in birth rates, shortened or absent breastfeeding, and increasing age at menopause, have contributed to this development . Differences in the duration of lactational amenorrhea have also contributed . Among segments of the halacha observant population, high fertility and breastfeeding rates still lead to months of amenorrhea due to pregnancy or lactation 
If there is no medical contraindication, using artificial means to adjust the menstrual cycle to prevent being niddah is halachically permissible. This strategy is commonly employed before marriage to prevent a woman being niddah at the time of her wedding (see Hormonal Cycle Manipulation for Brides). Couples may also wish to do this at other times, such as when planning a vacation.
If a woman is already using hormonal contraception for halachically approved reasons, then the adjustment is simple. She can be advised to continue the active pills for longer than three weeks as needed to prevent being niddah at the time that is important to her. If possible, it is best to make small adjustments over a number of cycles rather than a sudden major change. This minimizes the chances of breakthrough bleeding, which itself could potentially make her niddah or complicate the process of exiting the niddah status. The general use of extended regimens is tempting in this patient population. However, the advantages of reduced bleeding episodes  need to be balanced against the impact of unpredictable bleeding .
If the woman is not using contraception, a halachic question needs to be asked to determine whether the use of combination oral contraceptives is permissible in her case. The adjudicating rabbi will need to balance the halachic imperative of procreation against the difficulties caused by being niddah at an inappropriate time (e.g., halachic problems may arise if the couple is on a vacation while the wife is niddah). When choosing the formulation of estrogen/progesterone to be used, it is important to remember that breakthrough bleeding is more common when very low dose formulations are used . This is most common in the first months of use and thus it is better not to use the lower dose formulations for menstrual cycle manipulation, especially when only short term use is involved. Starting hormones one or two cycles in advance of the important event will minimize the chance of becoming niddah at the time most of concern to the couple. Couples should be aware of the minor and major side effects of the hormones (including the possibility that it will not work as planned) in order to make an informed decision.
Another possibility is to use progesterone only towards the end of the cycle to delay the onset of bleeding. The common drugs used are norethisterone acetate (Primolut-Nor) or medroxyprogesterone acetate (Aragest, Provera). This is generally given 5 days prior to the anticipated menses and continued as needed to delay menstruation. After about 10 days of use, the incidence of breakthrough bleeding goes up. Therefore if there is a need to delay the menstrual cycle for longer than 5-7 days, it is best to accomplish the delay by a smaller adjustment each month. As this is not a contraceptive method, the halachic difficulties of contraception are avoided. This is also appropriate when there is concern about the side effects of estrogen.
At times, physicians question the use of hormonal manipulation for such non medical considerations. However, this type of use is common  and felt to be ethically justified . Physicians should be aware of the major impact that being niddah can have on quality of marital life. If the wife does not have health issues that place her at greater risk, the risk benefit ratio would seem to justify providing this type of medical assistance.
Implications for Patient Care: If there are no medical contraindications, there is no halachic problem with manipulation of the menstrual cycle to prevent a woman being niddah at an inconvenient time. At times women will seek medical care for this purpose. Possibilities include:
- Extending the active hormones for women already on hormonal contraception
- Short term use of hormonal contraception
- Delay of bleeding by use of progesterone
When planning the regimen, it is important to assure not only that the woman will not be actively bleeding but that she will be halachically able to exit the niddah status in time.
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