Yom Kippur Fast - Nursing Mother
Question
A woman who gave birth at the beginning of the month of Elul and is fully breastfeeding (without any formula supplementation at all) is considering introducing formula on Yom Kippur so that she can complete the fast. Is her thought correct, or can she continue to breastfeed as usual and, if necessary, do as the Rabbi instructs her? Have a good year!
Answer
Greetings,
There is no need to give the baby a milk substitute; there is no point in doing that.
She should nurse as usual; she should drink a lot in the days before the fast, and rest in a cool air-conditioned place during the fast, and she should not lack milk, G-d willing.
If, in any case, the baby lacks milk, she should drink a large amount of liquid [not ‘measurements’], in order to restore lactation.
Source
Shulchan Aruch, section Orach Chaim, chapter 617