Theory:

In the previous theories we observed what happens when fertilisation occurs. So, what happens if fertilisation does not carry out? Let us take a look at what happens when fertilisation does not occur.
The cyclic event that takes place in a rhythmic fashion during the reproductive period of a woman’s life is called the menstrual cycle.
In females, the menstrual cycle starts at the age of \(11\)-\(13\) years, marking puberty and is called menarche. Menstruation ceases around \(48\)-\(50\) years of age, and this is termed menopause.
The reproductive period in females is marked by characteristic events that are repeated almost every month. In physiologically normal women, the cycle repeats once in \(28 \)days (with minor variation) in the form of a menstrual flow.
 
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Depicts menstruation
 
The beginning of the menstrual cycle marks the onset of puberty in human females.
The maturation of the immature oocytes into mature ova and subsequent events taking place in the female reproductive system is called the menstrual cycle.
One mature ovum is released (ovulation) from one of the ovaries during the middle of each menstrual cycle. Every month a matured ovum is released so that it can undergo fertilisation.
  
During the period, the uterus changes so it can provide nourishment for the fertilised egg. The uterine lining becomes thick and spongy for the implantation of a fertilised egg. If fertilisation occurs, the corpus luteum continues to secrete progesterone.
  
The secretion of progesterone maintains the thick uterine wall and prevents the maturation of another follicle till pregnancy.
 
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Releasing of the ovum from the ovary
  
If the ovum is not fertilised, the uterus sheds its lining. The corpus luteum degenerates, the egg disintegrates, and the uterus lining is discharged as blood and mucus.
  
Menstruation is the bleeding from the uterus, i.e., the shedding off of the endometrial lining of the uterus that causes bleeding.
 
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Menstrual cycle flow chart
 
The menstrual cycle consists of four phases:
 
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Stages of ovum
 
1. Menstrual or Destructive Phase
 
2. Follicular or Proliferative Phase
 
3. Ovulatory Phase
 
4. Secretory or Luteal Phase
 
Both the ovary and the uterus demonstrate synchrony of events during these phases. The pituitary hormones (LH and FSH) and ovarian hormones (oestrogen and progesterone) cause changes in the ovary and uterus.
 
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Menstrual cycle phases
 
A video about menstruation is provided below to understand the importance of the process:
 
Reference:
https://www.vecteezy.com/vector-art/1361046-scientific-medical-illustration-of-menstral-cycle-process
https://www.youtube.com/watch?v=ayzN5f3qN8g&t=31s
<a href='https://www.freepik.com/vectors/clock'>Clock vector created by pikisuperstar - www.freepik.com</a>
https://www.flickr.com/photos/euthman/3595010317/in/photostream/