It is forbidden to fast while menstruating. If a woman's period starts in Ramadan during the night (i.e. any time from the start of the Maghrib prayer to the start of the Fajr prayer), then she is forbidden from fasting the following day. She must continue to not fast for as long as she is menstruating.

However she must bring the lost fast after Ramadan, when she is in non-menstrual state.