How does a woman with an intermittent menstrual cycle determine her purity for fasting, especially if dryness continues for a day, then blood returns after ghusl and prayer, or after intercourse?

A woman performs ghusl (ritual bath) when she sees the sign of purity from menstruation. If the bleeding returns at a time when it could be considered menstruation, she must refrain from prayer and fasting. Then, she performs ghusl again when it ceases, and she does this every time it recurs. The period of purity interspersed within the menstrual cycle is valid, and she is not required to make up for acts of worship performed during it. So, if she sees the sign of purity before Fajr (dawn prayer), intends to fast, and fasts, then the bleeding returns after sunset, her fast is valid. It is permissible for a husband to have intercourse with his wife if she sees the sign of purity and performs ghusl, even if the bleeding returns thereafter.