Fasting while undergoing menstruation or post natal bleeding?
Dr Zakir Naik: As far as the woman who are undergoing menstrual cycles or post-natal bleeding according to our beloved Prophet Muhammad (saws), its mentioned in Sahih Bukhari, also in Sahih Muslim that it is forbidden for a woman to fast who is bleeding due to menstrual cycle or post-natal bleeding.

So it is not optional, it is forbidden. And the reason is because when the blood flows out in a lady, in a woman, she loses a lot of blood and on top of that if she fasts it will not be good for her health. So Allah (swt), it is His Mercy that he has exempted them and made it compulsory not to fast otherwise it may be damaging for her health. And Allah (swt) does not want to overburden any of the human beings. So that's the reason they are exempted from fasting and even in the middle of the day if the menstruation begins, they have to break their fast. Even if it begins a few minutes before sunset, they have to break the fast but they have to compensate later on. But if they are undergoing the menstrual period and if the period stops just before Fajr even if they did not have a bath, they can start their fast. But if it ends even a few minutes after Fajr, they cannot fast. They have to break their fast but they have to compensate it later on