B'sha'ah tovah!
According to many opinions, pregnant women are generally required to fast on
Tisha B’Av. However, permission to eat is given in situations in which the fast is a threat to the woman’s or fetus’s health. Given your concerns, we suggest you consult your physician prior to the fast and then get back to us or a local halachic authority.
A pregnant woman fasting on the ninth of Av should be sure to hydrate herself well beforehand and to get ample rest during the fast (lying down, arranging for babysitting, etc.) in a cool environment. If she begins to feel sick (beyond a light headache and lethargy), she can break her fast.
For more information on fasting during pregnancy or immediately postpartum, please see our article
here.