Start & End of Hayd
Menstruation is when uterine tissues break down and exit from the vaginal opening. The vaginal opening is enclosed by two sets of cutaneous folds – the labia minora, which directly cover the vagina, and the labia majora, which are the outermost folds of skin.
Click here for a diagram of the female anatomy.
The Fiqh of It
When blood exits from the vaginal opening, menstruation (hayd) has started. If a woman feels blood descending, but it does not reach the edge or exit from the labia minora, she remains pure. Similarly, if a woman blocks the blood flow, she will remain pure until the blood exits the labia minora. A woman will calculate her hayd from the moment she sees the blood. (Manhal al-Waridin pp.169-70)
There is no requisite in Islamic law (Shari’ah) as to how often a woman should check if her hayd has ended. However, she needs to ascertain whether she is pure at the end of the prayer time as that will determine if the prayer is obligatory on her. When her discharge becomes white or clear, or there is an absence of discharge, she is pure and should perform a ritual bath (ghusl) and return to her obligatory worship.
If she is not sure whether her hayd has finished, she can delay her ghusl to the end of the prayer – if there is no further bleeding, she should perform ghusl and pray. If she sees coloured discharge, she will leave the prayer and check again in the subsequent prayer time. (Manhal al-Waridin pp.200-1)
Using a piece of cotton fabric (kursuf) can be instrumental in determining the end of hayd and often eliminates the need for multiple ghusls.