Istimrar in a girl, that is, continuous flow of blood without it coming to a stop, may be classified in one of the four cases:

Rule 1: If the bleeding which is seen for the first time continues to flow without stopping, the first 10 days are considered menstrual and the next 20 days are considered days of purity.

Example:
If a girl sees bleeding for the first time in her life and it continues to flow without stopping, 10 days are considered menstrual and 20 days are considered istihada (non-menstrual bleeding). As long as blood flows, she will count 10 days as menstrual bleeding and 20 days as istihada.

EXPLANATION:

On seeing blood, she discontinues salat. When blood continues to flow on the 11th day as well, it is judged to be istihada. Thereupon, she must resume salat by taking a ghusl (purificatory bath). She should go by this pattern as long as blood flows. That is, she should consider 10 days menstrual bleeding and 20 days istihada.

Rule 2: If blood continues to flow without ceasing after a girl has experienced valid bleeding (that is, 10 days of bleeding at most) and valid purity (that is, 15 days of purity at least), this girl has become a woman with an established length of menstruation. Of these days of bleeding, the length of her previous period is considered menstruation and what exceeds it is considered istihada.

Example:
If a girl, for the first time in her life, bleeds for 5 days and then remains pure for 20 days and thereafter bleeding restarts and flows without stopping, she should consider 5 days menstrual and 20 days istihada. She should go by this pattern until the bleeding ceases. That is, she will count 5 days as menstrual bleeding and 20 days as istihada.

EXPLANATION:

On seeing blood, she discontinues salat and resumes it on the 6th day by taking a ghusl when the bleeding stops. She again discontinues salat when she sees bleeding on the 26th day because more than 15 days of purity have passed since the end of her last period. When she experiences bleeding on the 36th day, she should judge it to be istihada and resume salat by taking a ghusl. She must make up later the salats that she missed during 5 days of bleeding [31st, 32nd, 33rd, 34th, 35th days], which have been judged to be the days of istihada. She should go by this pattern as long as blood flows. That is, she should consider 5 days menstrual and 20 days istihada.

Rule 3: If a girl undergoes fasid bleeding (bleeding that lasts for more than 10 days) and fasid purity (purity that lasts for less than 15 days), neither of them is considered to be her established length of menstruation or purity.

This rule may be classified into three cases:
a. If it is fasid (flawed, vitiated) because purity period has lasted for less than 15 days, the bleeding which was seen first is considered to have flowed without stopping. A month normally refers to 30-day period. Therefore, if she has experienced fasid bleeding and fasid purity and the sum of the days of bleeding and purity has not exceeded 30, then the initial days of istimrar (continuous bleeding) are considered istihada (non-menstrual bleeding) and added to the sum of them to make it 30. Of 30 days of bleeding, 10 days are considered menstrual period and 20 days are considered istihada. She should go by this pattern until the bleeding ceases.

Example:
If a girl, for the first time in her life, sees bleeding that lasts for 11 days and thereafter remains pure for 14 days and then the bleeding resumes and continues to flow without stopping (that is, it has made istimrar), the bleeding she saw first is fasid as it has exceeded 10 days. Day 11 and as many initial days of istimrar as to total 30 are considered istihada. In the example above, 5 days, from the 26th day to 30th day, are considered istihada. After these 5 days, she will consider 10 days menstrual and 20 days the purity period.

EXPLANATION:

On seeing blood, she discontinues salat. When blood continues to flow on the 11th day, she should consider it istihada (non-menstrual bleeding) and resume salat by taking a ghusl. She again discontinues salat when she sees blood on the 26th day. When it continues to flow on the 36th day, that is, 11th day of the second month, she should resume salat by performing a ghusl as it may be istihada. When she notices that the bleeding does not cease but keeps flowing continuously (that is, it has made istimrar), she should consider the first 5 days [26th, 27th, 28th, 29th, 30th days] and 20 days after 10 days of menstruation to be istihada (non-menstrual bleeding). She will have to make up those salats that she missed during the days of istihada later on. She should go by this pattern as long as blood flows. That is, she will count 10 days as menstruation and 20 days as istihada.

b. If the purity is full purity and is fasid because there are days of bleeding within it and if the sum of the days of such fasid purity and the days of bleeding does not exceed 30, the initial days of continuous bleeding (istimrar) are considered istihada and added to the sum of them to make it 30.

Example:
If a girl, for the first time in her life, sees bleeding for 11 days and thereafter remains pure for 15 days and then bleeding starts and flows without stopping, there is istihada in the first bleeding as it has exceeded 10 days. Because she experienced bleeding for 1 day [the 11th day] before the 15 days of purity, 15 days of purity and 1 day of bleeding are considered fasid purity. If blood continues to flow without stopping, the first 4 days of it, in order to reach 30, are considered days of purity. Thereafter, she will consider 10 days menstruation and 20 days istihada.

EXPLANATION:

On seeing blood, she discontinues salat. When blood continues to flow on the 11th day as well, she should consider it istihada and resume salat by taking a ghusl. She discontinues salat on the 27th day as the bleeding has restarted. When blood continues to flow on the 37th day (that is, on the 11th day of the second month), it is considered istihada, so she should resume salat by taking a ghusl. She should go by this pattern as long as blood continues to flow. That is, she will consider 10 days menstrual and 20 days istihada.

c. If the sum of the days of bleeding and days of purity exceeds 30, the first 10 days are considered menstrual, and all the following days until the istimrar are considered days of purity. And after the istimrar, 10 days of menstruation alternate with 20 days of purity.

Example:
If a girl, for the first time in her life, sees bleeding for 12 days and remains pure for 20 days and thereafter bleeding starts and continues to flow without stopping, 10 days are counted as menstruation and 20 days are counted as istihada.

EXPLANATION:

On seeing blood, she discontinues salat. When she sees bleeding on the 11th day as well, she should consider it istihada and resume salat by taking a ghusl. She discontinues salat again on the 33rd day when she sees bleeding because more than 15 days of purity have passed since the end of her previous period. When blood continues to flow on the 43rd day (that is, 11th day of the bleeding of the second month), she should count it as istihada and resume salat by performing a ghusl. She should go by this pattern as long as blood continues to flow. That is, she should count 10 days as menstruation and 20 days as istihada.

Rule 4: If she undergoes valid bleeding (that is, bleeding that lasts for 10 days at most) and fasid purity (that is, purity period that lasts for less than 15 days), the days of valid bleeding become her menstrual period. The following days are considered days of purity until after the 30th day. That is, the initial days of istimrar are considered istihada and added to the sum of days of bleeding and purity until it is reached 30 days.

Example:
If a girl, for the first time in her life, bleeds for 5 days and then remains pure for 14 days and thereafter bleeding starts and continues to flow without stopping (that is, it has made istimrar), the first 5 days are considered menstrual and the following 25 days are considered days of purity. The first 11 days of istimrar are considered days of purity in order to reach 30. Thereafter, she should count 5 days as menstrual bleeding and 25 days as days of purity.

EXPLANATION:

On seeing blood, she discontinues salat. She resumes it on the 6th day by taking a ghusl when the bleeding stops. She should continue to perform salat on the 20th day, even though there is bleeding, because 15 days of purity have not passed yet. Therefore, it is certain that that blood is istihada. When blood keeps flowing on the 21st day, she should discontinue salat. When she sees bleeding on the 30th day (that is, 11th day of the bleeding of the second month), she should consider it istihada and resume salat by taking a ghusl. When it becomes clear that the bleeding continues to flow without stopping, the first 11 days (the 11 days after the 14 days of purity) and 25 days after 5 days of menstruation are considered istihada. She has to make up those salats that she missed during the days of istihada. She will go by this pattern as long as bleeding continues. That is, she will consider 5 days menstrual and 25 days istihada.