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Theory:

The beginning of the menstrual cycle marks the onset of puberty in human females. The menstrual cycle starts with the shedding off of the endometrial lining of the uterus that causes bleeding.
 
The thickening of the endometrium is due to the periodic preparation for pregnancy. Menstruation takes place when the mature ovum released by the ovary is not fertilised during ovulation.
Process of menstruation:
1. When a girl reaches puberty at about \(10\) to \(12\) years of age, sex hormones are released into the blood. The release of hormones causes the maturation of the ovum.
 
2. One mature ovum is released from one of the ovaries. The mature ovum is released into the oviduct or fallopian tube once every \(28\) to \(30\) days. This is called ovulation.
 
3. Before the process of ovulation, the inner wall or the endometrium of the uterus grows or thickens. The endometrium becomes spongy with full of tiny blood capillaries or vessels. This growth of the uterus is the preparation done to receive the fertilised egg or the zygote.
 
4. If the ovum is not fertilised, the endometrium of the uterus is shed. The endometrium is shed along with blood vessels and dead ovum from the vagina in the form of bleeding. This is termed menstruation.
 
5. Menstruation takes place \(14\) days after ovulation and usually lasts for about \(3\) to \(5\) days.
 
6. Once menstruation completes, the inner wall of the uterus once again builds up. It again gets ready to receive the subsequent ovum.
 
7. If the ovum is not fertilised even in this round, menstruation again takes place. Thus, the menstruation cycle is repeated again and again in the women every \(28\) to \(30\) days. The hormones control the menstrual cycle.
 
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Menstrual cycle flow chart
 
Menstruation stops temporarily when the ovum is fertilised by the sperm, which results in pregnancy. When fertilisation occurs, successful implantation of the fertilised egg takes place.
 
Menstruation halts during pregnancy so that the endometrium, which contains many blood vessels, supports the growth and development of fertilised ovum to form a baby.
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Menstrual cycle
 
Menstruation restarts after the birth of the baby. Menstruation also can stop due to:
  • Nutritional deficiencies
  • Low body weight
  • Stress
  • Eating disorder
  • Excessive weight gain
Reference:
https://www.vecteezy.com/vector-art/1361046-scientific-medical-illustration-of-menstral-cycle-process