The Grand Ayatollah vahid Khorasani answered a question about the rules of fasting for breastfeeding mothers.
Question: What is the ruling about a woman (mother) who breastfeeds a child?
The Grand Ayatollah Vahid: This question has 4 conditions;
The first condition: The woman (mother) knows that fasting has no harm to herself or the child, in this condition she must fast.
The second condition: Fasting is harmful only to herself and no harm to the child, in this condition;
1. Must not fast
2. Can fast later on for the lost days
3. The recommended (Mostahab) precaution is that she gives 750 grams of food to the poor for everyday (of fasting missed)
The third condition: Fasting has no harm for her but is harmful for the child in this condition;
1. Must not fast
2. Fast later on for those missed days
3. Must give 750 grams of food to poor for everyday (of missed fasting)
The fourth condition: Fasting is harmful to herself and for the child, this condition has the ruling mentioned for the second condition.
Note: The ruling for second and fourth conditions is possible only if giving milk to the child cannot be done in any other ways, and if it is possible that woman (the mother of the child) must fast.
source : Shafaqna