Regarding this issue, the eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states that ‘if a lady’s period starts before Maghrib adhan even if it was very close to the adhan of Maghrib, then her fasting has been broken and she has to make up for this day and the following days of her period later after Ramadan.
Yet, even if her fasting has been broken, once she gets menses before adhan, Almighty Allah will give her the reward for the fasting period she has done. So, the reward for her fasting before getting menses will not be lost.
Furthermore, it is good for whoever finds herself in this condition to know that she observed fasting due to Allah’s orders and her fasting has been broken upon getting menses based on Allah’s teachings and she is required to make up for this day later based on Allah’s commands. Therefore she gets a great reward for following Allah’s commands in each of the above actions.