Laws of Niddah for the Breastfeeding Woman
After postpartum bleeding (lochia) has ceased, many women do not menstruate for a number of months. For hormonal reasons, women who breastfeed generally resume menstruation later than those who do not. The halacha, however, does not depend on the woman's actual practice. Rather, during the first 24 months after giving birth she is considered a meineket (nursing mother), whether or not she actually breastfeeds. At the end of 24 months she is no longer halachically considered a meineket even if she continues to nurse.
A meineket has the halachic status of mesuleket damim (amenorrheic, without a monthly cycle). She is not required to anticipate her menses, and does not observe any onot perishah after the cessation of her postpartum bleeding, regardless of what her normal pattern was before she became pregnant. If, however, she does menstruate during this period, then the following month she observes the Hebrew date (yom hachodesh) as an onat perishah. After her second menses, she begins to observe the interval date (haflagah) as well.She should ask her rabbi whether to observe onah beinonit, as there are different opinions.
A meineket does not establish a regular cycle (veset kavua) even if she menstruates according to a consistent pattern. If she had established a veset kavua before becoming pregnant, then it is suspended during pregnancy and hanakah (nursing), and it is, in most cases, automatically reinstated 24 months after she gives birth.