Reference: Fataawa Ramadhaan – Volume 1, Pages 340-341, Fatwa No.287
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.10343
Question: A woman fasted and at sunset, just before the adhaan, her menses came. So does that invalidate her fast?
Response: If her menses came before sunset, then her fast is invalidated, and if it was after sunset, then her fast is valid and she is not required to make up [the fast for that day].
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.