Sheikh Muhammad Muhammad Sâlim `Abd al-Wadûd
A woman does not have to wake up at Fajr time during the last days of her menstrual cycle. She is not burdened with discovering the exact moment that her menstrual period comes to an end.
The women used to send samples of cotton to `A'ishah (may Allah be pleased with her) so she could judge for them when their periods came to an end. She would simply tell them not to be in such a hurry.
This shows us that showing an exaggerated concern for determining when the period comes to an end is contrary to the practice of our Pious Predecessors.
We also have the general axiom of Islamic Law that "certainty is not removed by doubt". In this case, the woman knows she is on her period. She is not under any legal burden to act upon the possibility that her period is over. Therefore, she is not required to wake up at Fajr time to determine whether her period might be over.
Likewise, we have the principle of Islamic Jurisprudence known as istishâb al-hâl. This means that in Islamic Law, the default assumption is that things are still as they have previously been. Since the woman is already on her period, she is free to continue to assume that she is on her period until evidence shows her that it is otherwise. She is not called upon to go out of her way and verify that she is still on her period. When it becomes clear to her that her period is over, then she should take her bath and resume her prayers.
The matter is one in which there is great flexibility and leniency. It should never be a source of worry or anxiety for any woman.
Source: Islam Today