(Mishnah): If Rosh Chodesh Kislev came and rain did not fall, Beis Din decrees three fasts on the Tzibur. They eat and drink after dark, and they are permitted Melachah, bathing, Sichah, wearing shoes and Bi'ah.

(Gemara - R. Yochanan): Every fast that Beis Din decrees to stop [eating before the end of the previous day], pregnant and nursing women do not fast on them.

(R. Shmuel bar Rav Yitzchak): This seems correct when they previously decreed [fasts] not to stop [so these women already fasted], but if they immediately decreed to stop, pregnant and nursing women fast on them.

Support (Beraisa): Pregnant and nursing women fast normally on Tish'ah b'Av, Yom Kipur, and the first three fasts, but on the second three fasts and the last seven fasts they do not fast;

Even so, they do not indulge themselves with delicacies, they eat and drink need to sustain the child (or fetus).

Is this not when they already decreed not to stop (then, when they decree to stop, pregnant and nursing women do not fast)!

R. Ba bar Zavda came, and said in the name of Rav Yitzchak bar Tavlei citing R. Yochanan, and even if they decreed immediately to stop, pregnant and nursing women do not fast on them.