The fast of Tisha B’Ab is treated more stringently than Shiba Asar Be’Tammuz, Asara Be’Tebet and Som Gedalya with respect to pregnant and nursing women. Whereas on the other three fasts Halacha exempts pregnant and nursing women from fasting, they are obligated to fast on Tisha B’Ab, and this is, in fact, the accepted practice.
The question arises, however, as to whether this applies even in years such as this year (5772), when Tisha B’Ab falls on Shabbat and is thus delayed until Sunday. (Fasting is forbidden on Shabbat, except when Yom Kippur falls on Shabbat.) Are pregnant and nursing women required to fast on Sunday, or is the fast treated more leniently in such a case?
Hacham Ovadia Yosef addresses this question in Yalkut Yosef – Ta’aniyot (p. 88; listen to audio recording for precise citation), and advances a "Kal Va’homer" rationale to allow pregnant and nursing women to eat on Tisha B’Ab in this situation. He notes that when a Berit is performed on a fast day, the three Ba’aleh Berit (the father, the Sandak and the Mohel) must fast despite the festive occasion; this applies not only on Tisha B’Ab, but even on Shiba Asar Be’Tammuz, Asara Be’Tebet and Som Gedalya. However, when a fast day falls on Shabbat and is observed the next day, and a Berit is performed, the Ba’aleh Berit are permitted to eat. Even on Tisha B’Ab, the Ba’aleh Berit may eat if Tisha B’Ab is observed on the tenth of Ab because the ninth is Shabbat.
Hacham Ovadia reasoned that if Ba’aleh Berit are permitted to eat on Tisha B’Ab in such a case, then this should certainly apply to pregnant and nursing women, as well. Halacha treats nursing and pregnant women more leniently with regard to fasting than Ba’aleh Berit, as evidenced by the fact that unlike Ba’aleh Berit, pregnant and nursing women are allowed to eat on Shiba Asar Be’Tammuz, Asara Be’Tebet and Som Gedalya. Thus, if Halacha allows Ba’aleh Berit to eat in the case of a delayed Tisha B’Ab, then certainly pregnant and nursing women may eat in such a case, as well. They may eat already in the morning, and it is possible that they may even eat already on Mosa’eh Shabbat, since they are exempt from the fast. Hacham Ovadia adds, however, that pregnant and nursing women in this case should not indulge in food and drink, and should instead eat and drink only as necessary for the wellbeing of the infant.
It should also be noted that a woman in this case must recite Habdala before eating, as Halacha does not allow eating after Shabbat until the recitation of Habdala.
Summary: Although nursing and pregnant women are generally required to fast on Tisha B’Ab, when Tisha B’Ab falls on Shabbat and is delayed until Sunday, they are allowed to eat and drink, though they should eat and drink only what is necessary for the infant’s wellbeing.