She did not make up for the Fasts due to fear for her breast-feeding Child
Question :
A woman gave birth in Ramadhan and she did not make up for the fasts after Ramadhan due to fear for her breast-feeding child. Then she became pregnant again and she delivered in the next Ramadhan. Is it permissible for her to distribute money instead of fasting?
Answer:
It is obligatory on this woman to fast in place of the days that she did not fast, even if it is after the second Ramadhan. This is because she only left off making up for the fasts between the first Ramadhan and the second Ramadhan. due to an excuse. I do not know would it be difficult upon her to make up for the fasts in the wintertime by fasting every other day. Even if she is breast feeding, verily Allâh will give her strength and that will not harm her or her child. Therefore, she should strive as much as she is able to make up for the fasts of the Ramadhan that passed before the next Ramadhan comes. If she is not able to do so, there is no sin on her in delaying it (making up for it) until the next Ramadhan.
Source:
Ash-Shaykh Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 3 Pages 302-303