Is it permissible for a breast-feeding woman to break her fast? And when should she make up for it? And may she feed (poor people instead of fasting)?
If the breast-feeding woman fears that the fasting will harm her child by reducing the milk, then she may break her fast, but she must make up for it later, because she is similar to a sick person, about whom Allaah says:
…and whoever is ill or on a journey, the same number [of days which one did not observe Saum (fasts) must be made up] from other days. Allaah intends for you ease, and He does not want to make things difficult for you. [al-Baqarah 2:185]
Then once the danger is past, she may make it up for, either during the winter, when the days are shorter and the weather is cooler, or, if she is unable to do it in the winter, then in the following year. But as for feeding (poor people), it is not permissible except in a situation where the thing which prevents her from fasting or the excuse for not fasting is continues, and it is not expected that it will be removed, in these circumstances, she may feed (poor people) instead of fasting.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Arkaanul-Islaam, DARUSSALAM, vol.2, page.643