“The breastfeeding mother by default is obliged to fast. However, if illness or harm is genuinely feared upon herself or the baby, only then is she exempted from fasting. She will have to do Qadhā’ later.
Genuine reason to omit fasting are ascertained by:
2) Previous experience
3) Clear and unmistakable signs” Mufti Faraz Adam al-Mahmudi, http://www.darulfiqh.com
“In essence, it is obligatory for a pregnant or nursing woman to fast in Ramadan as long as she does not fear harming herself, such as if she fears:
1) Falling ill
2) Worsening an existing illness
3) Delayed recovery from an existing illness
In the above cases it is permissible for her to break her fast.
Similarly it is permissible for her to break her fast if she fears for her child, meaning that, for example she fears that she will miscarry due to malnutrition or the milk will decrease (obviously something which you have explicitly disproved), thereby causing her child to fall ill or increase in illness.” Shaykha Umm Anas bint Adam
” [With regards to fasting in pregnancy,] the psychological effect of mother not fasting should not be ignored… advise healthy meals, dates, other slow release foods for suhoor – seeds etc, hydration and maybe a maternity vitamin supplement and give it a go. [And fasting whilst breastfeeding for] western women with western diet & reserves shouldn’t really cause any major problem…. hydrate between maghrib and fajr and eat healthy balanced meal with protein etc. not just fried food! Also suitable vitamin supplement and fresh fruit/veg & dates will help inshaAllah.” Dr. Umm-Safiyyah
“With regards to general advice, it is safe to fast in pregnancy. With regards to the advice re: preeclampsia, gest. diabetes, I don’t think you will find that the advice would be to fast- generally it would be very case specific.” Dr. Ahsan Alvi