There are two major fast days 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 pregnant women should fast on Yom Kippur and Tisha B’Av. Many poskim, including the rabbinic supervisors of this website, rule that a woman during a low-risk uncomplicated pregnancy should fast if she feels no unique difficulty. However, there are minority opinions that are more lenient, and each woman should consult her own halachic authority.
Medically, not much study has been done on the effect of fasting on pregnancy. One published article showed increased delivery by women the day after Yom Kippur. Another showed an increase during the end of Yom Kippur as well. In both studies, however, most of the deliveries were at term. There is not much evidence that fasting will cause preterm labor, at least in low-risk pregnancies. Thus, medical literature is consistent with the instruction that healthy pregnant women should fast. Even so, drinking plenty of fluids for a few days before the fast is recommended.
Halacha and medicine are both case-based. A pregnant woman should consult in advance with her physician about fasting and about when to break a fast. This is especially critical if she has any complications or risk factors. 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 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 pregnant woman should plan on spending the day in bed, resting 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).
