One of the most important principles in the rulings regarding menstruation is habit. A woman’s habit is the number of days she normally menstruates, as well as where in the month she experiences her bleeding. For example, a woman has a 5-day bleeding cycle, followed by 20 days of purity. We would say her habit in menstruation (hayd) is 5 days and her habit in purity (tuhr) is 20 days.
It’s completely normal to have fluctuations in one’s habit – it can change each month if the bleeding and purity remain in the allowed limits. For example, a woman bleeds 5 days, followed by 20 days of purity and her next menstrual cycle lasts 7 days, followed by 25 days of purity. These ups and downs are not only normal, and they do not effect the rulings.
Habit comes into play when a woman experiences irregular bleeding (istihada) or has an invalid purity following her hayd. For example, a woman who previously bled for 6 days bleeds beyond the 10-day maximum, for a total of 12 days. In this situation of irregular bleeding, she reverts to her habit and considers the first 6 days hayd and the remaining 6 istihada. (Manhal al-Waridin pp.182-3)
These simple examples highlight another crucial point – the accurate and consistent recording of instances of hayd and tuhr. Without knowing how long her previous hayd lasted, a woman will not be able to determine which days should be marked as hayd and which as istihada.
The concept of habit is also important to applying rulings related to purity. For example, a woman regularly has 25 days of purity between her menstrual cycles, but following her last hayd she continued to spot indefinitely. Her tuhr habit will determine how many days she should treat as istihada before considering the spotting hayd. (Manhal al-Waridin pp.211-2)
A woman should record the exact time her bleeding begins. She should monitor her bleeding throughout her hayd. When the bleeding ends, she should again note down the exact time. Keeping a log from cycle to cycle will help with determining rulings if any irregularities occur.