Menstruation (men-STRAY-shuhn) is a woman's monthly bleeding. When women menstruate, your body sheds the lining of the uterus(womb). Menstrual blood flows from the uterus through the small opening in the cervix and passes out of the body through the vagina. Most menstrual periods last from 3 to 5 days.
Menstruation (a period) is a major stage of puberty in girls; it's one of the many physical signs that a girl is turning into a woman.
When periods (menstruations) come regularly, this is called the menstrual cycle. Having regular menstrual cycles is a sign that important parts of your body are working normally. The menstrual cycle provides important body chemicals, called hormones, to keep you healthy. It also prepares your body for pregnancy each month. A cycle is counted from the first day of 1 period to the first day of the next period. The average menstrual cycle is 28 days long. Cycles can range anywhere from 21 to 35 days in adults and from 21 to 45 days in young teens.
In the United States, the average age for a girl to get her first period is 12. This does not mean that all girls start at the same age. A girl can start her period anytime between the ages of 8 and 15. Most of the time, the first period starts about 2 years after breasts first start to develop. If a girl has not had her first period by age 15, or if it has been more than 2 to 3 years since breast growth started, she should see a doctor.
When girls begin to go through puberty, their bodies and minds change in many ways. The hormones in their bodies stimulate new physical development, such as growth and breast development. About 2 to 2½ years after a girl's breasts begin to develop, she usually gets her first menstrual period. 
1. Prayer (Salah): It is haraam for a menstruating woman to pay both obligatory and naafil prayers, and they are not valid if she does them.
2. Fasting: It is haraam for a menstruating woman to fast, whether it is an obligatory or a naafil fast, and it is not valid if she does it.
4. Tawaaf al-wadaa’ (the farewell tawaaf) is waived in her case: If a female completes the rituals of Hajj and ‘Umrah, then she gets her menses before she goes home and that continues until she leaves, she may depart without doing the farewell tawaaf.
6. Intercourse : It is haraam for her husband to have intercourse with her, and it is haraam for her to allow him to do so.
7. Divorce : It is haraam for a husband to divorce a menstruating woman during her menses.
8. Requirement of Ghusl : When the menstruating woman’s period ends, she must do ghusl by purifying her entire body
When one of their women menstruated, the Jews would not eat with her or sit with her in their houses. The Companions of the Prophet Muhammad ) (May Allah Honour Him and Grant Him) peaceasked the Prophet Muhammad ) (May Allah Honour Him and Grant Him) about this, and Allah revealed the words (interpretation of the meaning): ‘They ask you about menstruation. Say: That is an adhaa (a harmful thing for a husband to have intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they have purified (from menses and taken a bath)…’ Qur’an.Surah Baqarah 2:222
The Prophet Muhammad (May Allah Honour Him and Grant Him peace)said: ‘Do everything except have intercourse.’ Word of this reached the Jews and they said: ‘What does this man mean, in every issue he differs from us!’" Sahih Al Muslim, 455
Menstruating woman and women who are bleeding following childbirth do not have to do the farewell Tawaaf. The disabled should do it, carried by others, as should those who are sick, because the Prophet Muhammad (May Allah Honour Him and Grant Him peace) said: “None of you should leave until the last thing that he does is to circumambulate the House.” And it was narrated in al-Saheehayn that Ibn ‘Abbas(R) said: Prophet Muhammad (May Allah Honour Him and Grant Him peace) told the people that the last thing they should do in Makkah was to circumambulate the Ka’bah, but he exempted menstruating women. Another hadeeth indicates that women who are bleeding following childbirth (nifaas) are like menstruating women and do not have to do the farewell Tawaaf.