Pregnant or Nursing Women and Tisha Be’Av Nidhe

One practical implication of Tisha Be’Av being pushed off to Sunday (“Nidhe“) is a leniency with regards to pregnant and nursing women. The Sha’arei Teshuva (O.H. 555:2) notes that a father whose son’s Brit Mila is on a pushed-off fast day is exempt from fasting. He notes that since pregnant or nursing women have the same status as the father, they are also exempt from fasting, as long as they have a little more discomfort than usual.
Rav Ovadia Yosef (Hazon Ovadia, Arba Ta’aniot, pg. 58)  proposes a novel rationale: On the remaining three public fast days (ie. Asara BeTevet, Shiva Asar BeTammuz and Tzom Gedalia), a pregnant or nursing woman is permitted to eat according to the Shulhan Aruch. Nevertheless, if a Brit Mila takes place on one of these fast days, the Ba’alei Habrit (the father of the child, the Sandak and the Mohel) are forbidden to eat. Since on a Tisha Be’Av Nidhe the Halacha is that Ba’alei HaBrit are allowed to eat, then surely pregnant or nursing women are exempt from fasting. 
Rabbi Ben Zion Abba Shaul (Or Letzion, vol. 3. 29:3) rejects this rationale, because it implies that a Tisha Be’Av Nidhe is more lenient than a regular fast day This is certainly not the case, as  the fast of Tisha Be’Av begins the night before and has restrictions that are not imposed on the other fast days. Therefore, he states that a pregnant or nursing woman must fast on Tisha Be’Av, whether or not it is pushed off. Most contemporary rabbis agree with this approach.
Nonetheless, if a woman has weakness or discomfort that is beyond what is to be normally expected during pregnancy, she would be exempt. As well, since Havdala will not be recited until Sunday night this year (5776), in theory one is not allowed to eat until that time. However, a pregnant or nursing woman may drink juice or the like even if Havdalawas not recited if she has any sort of discomfort. Even a pregnant or nursing woman who feels fine but is concerned that fasting may cause issues can drink, but it should it be consumed only in minimal amounts (less than a Revi’it and spaced in nine-minute intervals).
Summary:   When Tisha Be’Av is pushed off to Sunday, pregnant or nursing women who experience discomfort or have a medical condition are exempt from fasting.