Phases of Menstrual Cycle- Definition, Process & Function

Phases of menstrual cycle is an important part of the reproductive system. Students need to focus on this topic to understand the process of the reproductive system better. In females, the reproductive period starts from puberty and ends at menopause. A range of cyclic changes occurs in both the uterus and the ovary during this reproductive period. This cyclic change involves different phases of the menstrual cycle. The bleeding phase of the menstrual phase is one of the most prominent parts of the stages of menstrual cycle.

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Phases of Menstrual Cycle: Overall Details

The menstrual cycle involves a series of changes in the female reproductive part, i.e. ovary and uterus, that occurs in a cyclic manner with a periodicity of about \(28\) days, involving the shedding of blood for the first \(3-5\) days and ovulation on the \({\rm{14th}}\) day of the cycle. This process occurs in a few stages called the Phases of the Menstrual cycle. When the woman gets her first menstrual period, it is called Menarche. The permanent stopping of menstrual events is called Menopause.

Phases of Menstrual Cycle: Diagram

Fig: Phases of Menstrual Cycle

Phases of Menstrual Cycle: Details

The menstrual cycle occurs in four phases, i.e.,
a. Menstrual Phase
b. Follicular Phase
c. Ovulatory Phase
d. Luteal Phase

Phases of Menstrual Cycle: Process & Function

Menstrual Phase
1. This phase is also called the bleeding phase, which lasts for \(3-5\) days.
2. During this period, females shed their uterine blood out of their body through the vagina.
3. This bleeding is due to the breaking down of mucous membrane and gonads of the uterus lining called the endometrium.
4. If the secondary oocyte is not fertilized, it undergoes autolysis.
5. Progesterone, secreted by the persistent corpus luteum, inhibits LH from the pituitary.
6. Reduction of LH level causes regression of corpus luteum by autolysis and a consequent fall in progesterone in blood.
7. The basal part of the endometrium remains intact during the bleeding phase and acts as the source of new uterine lining and glands.
8. Lowered level of progesterone and oestradiol due to degeneration of corpus luteum causes the release of FSH from the adenohypophysis (anterior pituitary).
9. This initiates a new cycle.
10. In normal cases, this period of menstruation gets missed due to pregnancy. It may also get missed due to some internal problems of the body or any medication, stress, etc.
11. Menstruation occurs due to the absence of fertilization, i.e. if the ovum is not fertilized.

Follicular Phase
1. This phase lasts for \(9-10\) days, i.e. from the \({\rm{5th}}\) day to the \({\rm{14th}}\) day.
2. There occurs formation or repair of the damaged endometrial lining of the uterus. This repair is to receive the fertilized egg.
3. Reduction in ovarian and gonadotropin hormones stimulates the production of GnRH of the hypothalamus.
4. There is a subsequent increase in gonadotropin from the pituitary gland.
5. FSH stimulates the production of Oestrogen from the ovarian follicles.
6. Oestrogen also maintains the thickness of the endometrium by stimulating its proliferation.
7. One of the follicles, from about \(2,00,000\) to \(4,00,000\) primary follicles, matures during the phase and forms Graafian follicles. This happens due to the high level of FSH.
8. During this period, the primary oocyte is changed to a secondary oocyte. There occurs meiotic division to change the primary oocyte to the secondary oocyte.
9. Ovarian follicles begin to secrete estrogen. Its level increases three days before ovulation.
10. More thickening of the endometrium takes place due to the influence of estrogen. Thickening of the endometrium involves richly supplied blood vessels and endometrial glands.

Fig: The Phases of Menstrual Cycle and Various Changes.

Ovulatory Phase
1. This phase is marked by ovulation which occurs on the \({\rm{13th}}\) or \({\rm{14th}}\) day. Ovulation means the release of a secondary oocyte from the Graafian follicle, which after release, travels down the oviduct.
2. Higher levels of both LH and Oestrogen causes rapid growth of Graafian follicles and radial elongation of the corona cells.
3. This weakens the attachment between corona cells and the rest of the follicular cells.
4. Graafian follicle rises to the surface of the ovary and produces a stigma or protuberance.
5. This stigma later ruptures, and the ovum surrounded by corona radiata comes out in a viscous fluid.
6. Ovulation occurs roughly in the middle of the Menstruation Cycle.

Luteal Phase
1. This phase is also called the Secretory phase/Post-ovulatory phase.
2. It starts after the \({\rm{14th}}\) day and continues till the \({\rm{28th}}\) day, i.e. it occurs for \(12-14\) days.
3. During this phase, the lining of the uterus thickens further.
4. The increase in LH level in the blood is called LH surge.
5. This LH surge also leads to the transformation of the empty Graafian follicle to the Corpus luteum.
6. The follicular cells are converted into luteal cells by deposition of yellowish lipid inclusions. This is called luteinization.
7. Ruptured Graafian follicles are now called Corpus Luteum.
8. Progesterone is secreted from Corpus Luteum.
9. The small amount of Oestrogen is also secreted by this. The theca cells also release some androgens.
10. Both LH and progesterone help in the maintenance of the endometrial lining. The thickness becomes \({\rm{5 – 6\; mm}}{\rm{.}}\)
11. Progesterone is essential during pregnancy, and its level is high during the whole gestation period.
12. This progesterone is essential for the maintenance of the endometrium.
13. In this thickened endometrium, implantation of blastocysts occurs.
14. Progesterone affects the mammary gland also.
15. Progesterone does not allow the further maturation of any new follicle. This inhibits the uterine movement as well.

Fig: Menstrual Phases and levels of hormones.

What Happens if the Ovum is not Fertilized?

If there is no fertilization, then increased levels of progesterone inhibits the secretion of GnRH, i.e., Gonadotropin Releasing Hormone, which in turn reduces the secretion of FSH and LH. This results in degeneration of the corpus luteum to a white body called corpus albicans which in turn reduces the level of progesterone and estrogen.

This low level of hormones results in constriction of blood vessels, resulting in no supply of nutrition and oxygen to the endometrium. This results in the commencement of shedding of blood vessels or menstruation. Along with the blood vessels, the egg is also discharged.

Phases of Menstrual Cycle – At a Glance

Summary

The menstrual cycle occurs in four phases, i.e., Menstrual Phase, Follicular Phase, Ovulatory Phase, Luteal Phase. This cycle goes on for \(28\) days, after which a new cycle begins. For \(3-5\) days, shedding of blood occurs. All the phases of the menstrual cycle are well regulated by hormones like FSH, LH, Progesterone and Oestrogen. On the \({\rm{1}}{{\rm{4}}^{{\rm{th}}}}\) day, ovulation takes place. If fertilization happens then, menstruation gets halted for nine months during pregnancy.

But if fertilization does not occur, then menstruation occurs, and corpus luteum, which keeps intact during gestation, gets degenerated in the absence of fertilization to corpus albicans. Corpus luteum releases progesterone during pregnancy. From menarche to menopause is considered as the reproductive age of a female.

Frequently Asked Questions (FAQs) on Phases of Menstrual Cycle

Frequently asked questions related to phases of menstrual cycle are listed as follows:

Q1. What are the phases of Menstrual Cycle and its function?
Ans: The phases of menstrual cycle are: Menstrual Phase, Follicular Phase, Ovulatory Phase, and Luteal Phase.

Q2. What are the three stages of menstrual cycle?
Ans: Three stages of the menstrual cycle are:
a. Menstrual phase
b. Follicular phase
c. Luteal Phase

Q3. Explain Phases of Menstrual Cycle process with diagram?
Ans: a. Menstrual Phase- During this period, females shed their uterine blood out of their body through the vagina.
b. Follicular Phase- There occurs formation or repair of the damaged endometrial lining of the uterus.
c. Ovulatory Phase- Ovulation means the release of a secondary oocyte from the Graafian follicle, which after the release, travels down the oviduct.
d. Luteal Phase- During this phase, the lining of the uterus thickens further.

Q4. What phase of the cycle is menstruation?
Ans: Menstrual phase is the phase in which menstruation occurs.

Q5. What are the two major phases of the menstrual cycle?
Ans: Follicular Phase and Luteal Phase are the two major phases of the menstrual cycle.

Q6. Ovulation takes place on which day of a menstruation cycle?
Ans: Generally, ovulation takes place on the \({\rm{14th}}\) day of the menstrual cycle.

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