Hazrat Mujahid (may Allah have mercy on him) says: If a woman is observing her waiting period ('iddah) through menstruation and her menstruation stops, then she should resume her 'iddah by counting months. And if a woman is observing her 'iddah by counting months and menstruation starts, then she should resume her 'iddah through menstruation.
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2457