Fasting for a Woman who has Given Birth and is Nursing
Question
My wife, thank G-d, gave birth on the 17th of Elul. She is fully breastfeeding. Is she obligated to fast as usual?
Answer
Shalom and Mazal Tov!
She needs to fast.
However, she should be careful to drink a lot beforehand, and during the fast, she should rest and stay in an air-conditioned space so as not to become weak.
If she feels the symptoms of dehydration, such as strong dizziness, blackness in the eyes, etc., she should drink ‘by measurements’.
In order not to reduce breastfeeding, she should drink properly in the days before the fast. However, if during the fast there is a lack of food for the baby, she should immediately drink a large amount of liquid.
Source
Shulchan Aruch, Orach Chaim, 617