When a woman sees her monthly period, she must stop fasting, and even if she fasts, she has no reward for such fasts. Hence, she must make up the days she missed during which she menstruated in Ramadan when she becomes pure. This is because of the Hadith of Aisha (may Allah be pleased with her), she said: “That happened to us, that is menstrual period, during the time of the Messenger (peace be upon him), and we were ordered to make up for the fast, but were not ordered to make up for the prayer.” Transmitted by Bukhari.
Making up the fast by the menstruating woman may take place immediately (after Ramadan) or longer after that time. The inability of a woman to fast for her long past years, she must calculate the number of fasts she has missed for those years. If she can not recollect the exact missed days, she can make an estimation for those years she was obligated to fast, (till she feels the days have covered more than expected).
About paying the expiation for negligence with making up for all her burden, which is feeding a poor person each day, as a reason for delaying the makeup without an excuse until the following year’s Ramadan; there are scholars who supported this, such as An-Nakha’i, Abu Hanifah and his students. The author of ‘Ar-Rqwdatu Nadiya‘ suggested that nothing is necessary except to make up only. In any case, the issue is controversial. If the woman wants to pay an expiation for negligence, then she can, but if she is satisfied with just making it up, she has this too.