There are two major fasts on the Jewish calendar. Unlike the other fasts, which last from dawn to dusk, these fasts are a little over 24 hours long, and their rules are more stringent.
The fast of Yom Kippur is a Torah obligation (Vayikra 16:29 and 23:27) and is the most stringent of the fast days. Tisha B’Av marks the anniversary of the actual destruction of both Temples. Therefore, its halachot are more stringent than the other rabbinically-ordained fasts, but less so than those of Yom Kippur.
There are different opinions regarding the exact circumstances under which breastfeeding women should fast on Yom Kippur and Tisha B’Av.
Many poskim, including the rabbinic supervisors of this website, rule that healthy women nursing healthy babies without problems should fast on both major fast days. However, there are minority opinions that are more lenient, and each woman should consult her own halachic authority.
Note that there are different opinions regarding Tisha B’Av nidcheh, when the fast is postponed from Shabbat to Sunday. We follow the position that a healthy woman nursing a healthy baby should attempt to fast on a Tisha B’Av nidcheh. However, the fast should be treated with greater leniency than in other years, and she may begin drinking or eating as necessary at the first sign of feeling unwell. She should recite havdala before eating or drinking anything other than water.
Medically, there has been little study to date of the effect of 24+ hour fasts on breastfeeding. The first published article showed minimal biochemical changes in the milk and a small reversible decrease in milk supply on the day following a 24-hour fast, which is consistent with the instruction that healthy women who are easily nursing healthy babies should fast.
Halacha and medicine are both case-based. Therefore, if a woman is not feeling well, or she is having difficulty breastfeeding, or the baby is very young or even mildly ill, or if there are concerns about milk supply, or psychological concerns, or any other unique difficulty, a woman should consult with her physician about fasting and about when to break a fast. It is important that the physician (even if not religious or non-Jewish) be sensitive to the seriousness of this fast and give a well-considered response. The woman should then discuss any medical concerns with a halachic authority.
Because fasting on Yom Kippur is a Torah obligation, it takes precedence over attending synagogue. A woman’s fasting on Yom Kippur even takes halachic precedence over her husband’s attending synagogue. On Tisha B’Av as well, fasting takes precedence over either spouse’s synagogue attendance.
If it will help her to keep the fast, a breastfeeding woman should plan on spending the day in bed, resting, nursing, and praying on her own.
A woman who needs to break her fast should have in mind while eating that she is fulfilling the mitzva of “v’chai bahem,“ living for the sake of Torah (and not risking life).
