Pregnant and nursing women are obligated to fast on Tish’a B’Av, because only the infirm are exempt from that fast, and pregnant and nursing women are considered healthy, unless they feel unusually weak. These women, however, need not fast on the minor fast days, for the following reason. According to the letter of the law, the Prophets ordained that we observe these fasts when Israel is faced with harsh decrees, but when no such decrees exist, it is up to the Jews to decide whether they want to fast or not. And indeed, the Jews have accepted upon themselves to fast on these days until the Temple is rebuilt, speedily in our days. However, from the very beginning, the custom has been that pregnant and nursing women do not fast on these days, because it is harder for them to fast.
In Germany (Ashkenaz), many pregnant and nursing women had a custom to act strictly and fast on the minor fast days. Perhaps they did so because of the harsh decrees that the Jews suffered there. In any event, the prevalent custom today, even among Ashkenazi Jews, is that pregnant and nursing women do not observe the minor fast days. And even if a particular woman wants to act stringently, it is preferable that she not fast if she has a hard time doing so. From the moment a woman knows she’s pregnant, she is exempt from the fast.
A nursing woman is exempt from the minor fasts as long as she nurses her child. Even if the child receives additional nourishment, the mother need not fast as long as she has yet to stop nursing her baby. Some poskim exempt all women from fasting for 24 months after giving birth, because [in their opinion] the exemption does not depend on nursing but on the hardships of childbirth, from which it takes 24 months to recover. In practice, most poskim rule strictly and require every woman who has stopped nursing her child to fast even on the minor fast days. This is the prevalent custom, but one who wants to adopt the more lenient opinion has upon whom to rely, for several great poskim rule leniently on this issue.
Rabbeinu Yerucham, Radvaz, and other poskim write that pregnant and nursing women are forbidden to fast on the minor fast days (see Torat HaMo’adim 2:2). The Rama writes that such women have a custom to act stringently and fast. The Acharonim (CA 133:6, AHS 550:1) explain that healthy women have the custom to act strictly, while those who experience a little distress need not follow this custom, but they are allowed to fast; and those who suffer greatly are forbidden to act stringently. Nowadays, the prevalent ruling for Ashkenazi women is not to fast. See Piskei Teshuvot 550:1, which cites authorities who are extremely lenient, saying that all women who are capable of giving birth are exempt from these fasts, so that they can have strength to give birth. [Piskei Teshuvot concludes that] some say that [such women] should “redeem” the fast with charity. Most poskim do not accept this opinion, but one may use it as an additional reason to rule leniently in questionable circumstances.