Be-Sha’ah tovah!
Healthy pregnant women are required to fast on Tisha B’Av unless there are specific health concerns. Please consult with your physician, and then discuss any medical concerns with a halachic authority.
There are different opinions regarding a Tisha B’Av nidcheh, when the fast is postponed from Shabbat to Sunday. We follow the position that a healthy pregnant woman should attempt to fast on a Tisha B’Av nidcheh, but that the fast should be treated with greater leniency. For example, if you start to feel at all unwell during the fast, you may begin drinking or eating as necessary. Just be sure to recite havdalah first.
Ideally, you should prepare before the fast by drinking extra water for a number of days leading up to the fast. Try to rest during the fast as much as possible, staying indoors in a cool room and (if relevant) making arrangements for help with childcare, or having your husband stay home as necessary.
Please get back to us with any further questions.