Fasting during pregnancy or nursing?
Dr Zakir Naik: According to a verse of the Quran in Surah Baqarah ch. no. 2 verse no. 185 that

"If a person is ill and traveling he can make up his prescribed periods of day of fasting later on"

So most of the Fuqas, most of the scholars, they say that these women who are pregnant and breast-feeding, they come under the category of those who are ill. So therefore if they want they can abstain from fasting but there are clear-cut Sahih Ahadith in which the Prophet has exempted these women from fasting mentioned in

Sunan Ibn Maajah Hadith no. 1667 that our beloved Prophet says

"The obligation for a person to fast and part of the prayer has been lifted from a traveler. And the obligation for fasting has been lifted from a lady whose pregnant and who is breast-feeding"

Our beloved Prophet Muhammad (saws) also said its mentioned in

Sunan Nasai in the Book of Fasting Hadith no. 2274 that

"Allah (swt) has relieved the traveler from fasting and half his prayer and Allah (swt) has relieved the pregnant woman and the woman who is breast-feeding from fasting."

So based on these 2 hadith also, its clear cut that the pregnant woman and the women who are breast-feeding, they've been exempted and the logical reason that one can think is because when a lady, she's pregnant or she is breast-feeding the food she eats is not only for herself, its for herself as well as the baby she is carrying or the baby she is feeding.
So but natural if her fasting makes it difficult for her or it becomes difficult for her baby then she is exempted from fasting. But if she is in her early stage of pregnancy and if she feels or if she is breast-feeding and if she feels the fasting will not cause any damage to her health and will not cause a damage to the baby then she should fast. If she has a doubt, if she can do it with hardships as long as there is no damage to herself, to her health and the baby she has an option to fast or not to fast. But if it's causing damage to her health or to the baby, it becomes haraam for her to fast because Allah (Swt) does not want to lay a burden more than what a person can bear. And in this regard that's the reason Allah has exempted them.

Later on when they are healthy and they stop breast-feeding or when the pregnancy ends after post-natal bleeding may be then they have to make up for the fast they have missed before the next Ramadhaan.