Nisaa
The scholars unanimously agree that any Syam during menstruation is invalid, whether it is obligatory or voluntary, and that it is not permissible for menstruating women to fast.
This is based on the hadith of Abu Sa’eed al-Khudri in which the Prophet (Salla Allahu 'Alayhi Wa Sallam) said, "Is it not the case that when she menstruates she does not pray or fast?" They replied, "Yes." He said, "That is her shortcoming in religious commitment." (narrated by Muslim).
Therefore, if a woman begins menstruating at any time during the day, her fast is invalidated.
Furthermore, the scholars agree that she must make up missed obligatory fasts from Ramadan, as 'Aishah (Radi'Allahu Anha) stated regarding menstruation: “When we would have our menses during the lifetime of the Prophet (Salla Allahu 'Alayhi Wa Sallam), we were ordered to make up the days of fasting that we had missed but were not ordered to make up the prayers that we had missed.” (narrated by al-Bukhari).
If a woman gets her period while fasting, her fast is invalidated, even if the bleeding starts just before sunset. She must make up for that day if it was an obligatory fast. It is not allowed for her to continue fasting during menstruation.