Answered by Shaykh Yūsuf Badāt
Do the 6 fasts of Shawwāl qualify as making up the fasts a woman missed due to menses, if she makes that intention?
In the Name of Allāh, Most Merciful, Most Compassionate
Thank you/ Jazāk Allāh Khayr for your question.
Ramaḍān make up fasts are mandatory (farḍh). If a believer fasts six fasts in Shawwāl as make-up (qaḍhā) or vow (nadhr), it is hoped from the mercy of Allāh that one also attains the virtue and reward of fasting six days in the month of Shawwāl.
It would be better to perform the make-up (qaḍhā) fasts exclusively and perform the six fasts with an exclusive intention. (Mughnī al-Muḥtāj)
And Allāh Knows Best