Prescribed fasts are those which a Muslim is obligated to complete in the month of Ramadan. However, if a woman is menstruating (hayd) or has post-natal bleeding (nifas), the obligation is lifted and she will make up (qada) the missed fasts once she is pure. On the other hand, irregular bleeding (istihada), does not invalidate the fast and she can continue fasting.
If a woman enters hayd or nifas during the day in Ramadan, her fast is broken. She should not abstain from food and drink as that would result in her resembling the fasting person. She should exercise discretion and refrain from eating in public; however, she is free to eat and drink in the privacy of her own home. If she becomes pure during the day in Ramadan, she should abstain from further food or drink, due to the sanctity of the month. It will still be necessary for her to make qada of the fast. (Hashiya al-Tahtawi 678/1)
It is recommended that she make up the fasts before the next Ramadan. Although there is no penalty in delaying, she should discharge this obligation as soon as possible (Bada’i al-Sana’i 104/2). She can fast consecutively or intermittent days. For example, if a woman missed 6 days of fasting during Ramadan due to hayd, she can fast 6 days in a row, or she can fast once or twice per week until she has fulfilled her obligation.
If a woman enters hayd or nifas while carrying out a nafl fast, her fast will be invalidated and must also be made up. Voluntary acts of worship, which are initiated from a person’s own volition, once started, must be completed. For example, if a woman undertakes a voluntary fast on Monday and begins menstruating before Maghrib, she will be obligated to make qada of the fast when she is pure. As she opted to fast voluntarily, she is obligated to carry it out from start to finish.