My wife be 43 yrs old, this is our first baby, we have no precedent in terms of knowing how she will handle a fast, how the fast might impact her production of milk, and implication for our daughter.
Pumping milk ahead of time(which she has begun to try to do, but the baby is only a month old b”h, and has not taken a bottle up til now so not even sure this is shayach….
Formula is not a possibility for the beby
What are the parameters for how and when she should decide that she needs to eat or drink? What are the symptoms to determine if and when she should eat? And what amounts should she eat and drink, and how frequently?
If she is feeling weak, or she sees that the baby is not getting the nourishment she needs, should my wife eat a full meal? What factors establish pikuach nefesh for my wife or for the baby?
Hearty Mazal Tov for the birth!
In general the parameters for nursing is piku’ach nefesh. Meaning, it is permitted to eat or drink on Yom Kippur only when a life might be endangered. Nursing does not usually fall into this category, and the Shulchan Aruch (617:1) rules that nursing mothers must fast.
For a nursing baby, experience shown that in the great majority of cases it is enough for a mother to ensure that she is well hydrated before the day, meaning drinking plenty several days in advance. She should also rest during the fast, if necessary with her husband’s help.
This will ensure that she has sufficient milk for close to the end of the fast.
In addition, as you note, she can pump and try to get the baby used to a bottle in advance.
Only under extreme circumstances – where the mother has run out of milk and the baby won’t take any alternative – will it be permitted to eat and drink. In this case the mother should do so (if possible) in small amounts (for instance of grape juice and sweet food) every nine minutes.
Good luck and best wishes.