What is the ruling if the woman fasts while she is uncertain of her period?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.

The menstruating woman must not fast; this is agreed upon by all Muslim scholars through different ages. None has ever said the contrary. If a woman does the contrary, she does what is against Sunnah. She should remember what Allah says:

“And whatsoever the Messenger (Muhammad (gives you, take it; and whatsoever he forbids you, abstain (from it)” Al-Hashr: 7

All authentic evidences prove that a menstruating woman should not fast.

Aisha RAA said: “When I had to make some days of Ramadan up because I did not fast, I was able to do so only in Sha’ban (the eighth month of Muslim Calendar)”

The prophet also said: “Isn't it true that a woman does not pray and does not fast on menstruating? And that is the defect (a loss) in her religion."

It is also narrated that Mu'adha al-‘Adawiya said that when she asked ‘A'isha why one who has been menstruating must make up for her fast but not for her prayer, she replied, “That happened to us, and we were ordered to make up for the fast, but were not ordered to make up for the prayer.”

Therefore, the woman should make up the same number of days when she does not fast due to a valid excuse, but she doesn’t have to make the prayers up.

And if for a reason, the woman has to fast two months in a row, but she has her period, she can continue her fasting after the period, and she does not have to start over again.