As we have stated, pregnant and nursing women are obligated to fast on Yom Kippur (Pesaḥim 54b; SA 617:1). True, some contemporary poskim maintain that nursing women may drink le-shi’urim, so that the fast does not cause them to stop nursing. Nevertheless, according to most poskim, nursing mothers must fast on Yom Kippur and even Tisha Be-Av. Even though nursing makes the fast more difficult because of the additional loss of fluids, this is not life-threatening. It does not endanger the baby either, because even if the mother’s milk decreases or dries up, milk substitutes are available. In reality, women who fast are generally able to continue nursing.
Good advice for nursing mothers is to skip every other feeding. This will help them make it through the fast relatively easily. In other words, a woman who normally nurses every three hours should nurse at 10 AM. At 1 PM she should use formula or another substitute, and then nurse again at 4 PM. At 7 PM she should once again use a substitute. This way, she will not suffer too much during the fast, and she will not produce less milk. Some babies will not accept a substitute from their mother, in which case someone else needs to feed them formula.
Nevertheless, if the baby is small, weak, and sickly, and the doctor thinks that the baby has a special need for mother’s milk, and there is concern that if the mother fasts, her milk will dry up or be considerably depleted, she may drink le-shi’urim based on the instruction of a God-fearing doctor (BHL 617:1). However, this is rare; if a nursing mother drinks a lot the day before the fast, her milk will almost certainly not be depleted from fasting. It is even better if, starting three days before the fast, she drinks and sleeps more than usual. This will increase her milk supply. She can also pump in the days before the fast, so the baby will have plenty of milk during the fast, and there is no concern that the mother’s milk will be depleted.
Furthermore, in the past, infant mortality during the first year was very high, and no good milk substitutes were available. Yet the clear ruling was that nursing women were obligated to fast, even on Tisha Be-Av. How then can we even raise the possibility that nowadays this is life-threatening? Good substitutes are available, and we have never heard of a baby who died because his mother stopped nursing him. Even in the past, when there were no milk substitutes, leniencies were granted only in cases where the baby was sick. We see this in Devar Shmuel §107 (cited in Responsa Ḥatam Sofer 6:23); BHL 617:1; Da’at Torah ad loc.; and Har Tzvi (OḤ 1:201:1 toward the end). Therefore, a nursing mother may drink on Yom Kippur only if there is a specific medical reason that her sick baby needs mother’s milk, and there is a concern that fasting will deplete her supply. Torat Ha-yoledet (ch. 51 n. 11) and Piskei Teshuvot (617:2) incline this way as well.
As we stated, the leniencies stem from times when there were no milk substitutes. Back then, if a mother’s milk dried up, she had to hire a wet nurse. If she did not have money for this, the baby’s life was truly in danger due to the likelihood of malnutrition. Today, milk substitutes are available, so it would seem reasonable to be stringent even with sick babies. In practice, though, the ruling is made by a doctor. If the doctor feels that there is a certain danger to the baby’s life if he cannot have mother’s milk, and there is a reasonable fear that the mother’s milk will dry up as a result of the fast, one can rule leniently. However, if the doctor is one of those who instructs many nursing mothers to drink le-shi’urim on Yom Kippur, then most poskim say one cannot rely on his judgment. Rather, one should ask a doctor who proceeds from the assumption that stopping nursing is not normally life-threatening.
We must add that the possibility that a mother’s milk will dry up due to the fast is remote. Generally, fasting does not force a woman to stop nursing. That is, if one is careful, in the days before the fast, to drink at least three liters a day and sleep for eight (or at least seven) hours a day, it is very unlikely that her milk supply will be depleted. On the contrary, many women report that proper preparation for the fast improves their milk supply; they discover how helpful extra drinking and sleeping are for nursing. Moreover, even if a nursing woman did not prepare properly for the fast, if she drinks and rests a lot after the fast, her milk supply will generally return to normal. (Nevertheless, if a nursing woman is in the process of reducing the number of feedings, or has trouble nursing in general, lack of proper preparation for the fast may make it hard for her to replenish her milk supply.)
To sum up, the Torah commands us to fast on Yom Kippur. The Gemara and poskim explicitly state that a nursing woman is included in this obligation. (Even on the rabbinic fast of Tisha Be-Av, pregnant and nursing women are required to fast.) There is no justification for not observing this mitzva based on the unsupported claim that it is life-threatening for nursing women to fast.