Menstrual Cycle: Definition, Phases & Hormonal Regulation

Did you know the period of reproduction in primate females is very well marked? The beginning of the menstrual cycle represents the onset of sexual maturity and reproductive cycles in females, while termination of the menstrual cycle also marks the termination of reproductive stages in females. The menstrual cycle is a part of reproductive events in primate females (apes, humans, etc.). The menstrual cycle occurs every month and involves a series of events that includes the discharge of menstrual blood from the female body, development of the ovum (female gamete), and formation and degradation of the endometrial lining. Read this article to learn more about the menstrual cycle, its stages, hormonal changes, and more.

Definition of Menstrual Cycle

Menstrual Cycle refers to the series of cyclic changes that occur in the reproductive system of the primate female’s body every month, to be precise \(28\) days. The breakdown of the inner, thick soft lining of the uterus along with its blood vessels in the form of vaginal bleeding is called menstrual flow or menstruation. This menstruation is repeated again and again in women after every \(28\) days.

Is Menstrual Cycle And Oestrous Cycle Same?

No, there is a remarkable difference between the menstrual cycle and oestrous cycle. The menstrual cycle is the term related to cyclic changes that occur in the reproductive system of the primate female’s body every month, while oestrous cycle is the term used to describe cyclic changes that occur in the reproductive system of the non-primate (cow, sheep, buffalo, etc.) female’s body every month.

What is Menstrual Cycle?

1. Blood linings in the uterus become thick side by side when eggs and follicles are developing in the ovary. 
2. This preparation in the uterus is done to receive an egg that may develop into a child.
3. There occurs shedding of the blood linings of the uterus (if it does not receive an egg) out through the vagina during menstruation for \(3-5\) days. 
4. This menstruation occurs \(14\) days after ovulation.
5. First menstruation starts at the age of \(10-13\) years in girls after they attain puberty.
6. The cycle which starts from one menstruation till the next one is called the menstrual cycle.
7. The menstrual cycle may be temporarily stopped only by pregnancy in normal cases.

Menarche and Menopause

1. Menarche refers to the onset of menstruation in a young female at about the age of \(10-13\) years. It is the beginning of the reproductive life of a girl.
2. Menopause refers to the permanent stoppage of menstruation at about the age of \(45-50\) years. At this age, a woman can no longer get pregnant.

Duration of Menstrual Cycle

1. The duration of the menstrual cycle varies from woman to woman. 
2. Ideally, the duration of one menstrual cycle in a normal female consists of about \(28\) days.
3. Due to variation in life styles and hormonal variance, the duration of the menstrual cycle can vary from \(21-40\) days or more.

Regular and Irregular Menstrual Cycle

Did You Know?

In a normal reproducing female, the menstrual cycle may be deferred or absent for a brief duration. These exceptions are as follows:

1. Pregnancy: Once the pregnancy is established, the menstrual cycle is stopped to facilitate the development of the embryo and, later, the foetus.
2. Lactational amenorrhea: During the period of lactation i.e., a period when the female feeds the young ones after childbirth or parturition, there is the absence of a menstrual cycle for a maximum period of about \(6\) months. This period is called lactational amenorrhea.

Phases of Menstrual Cycle

The menstrual cycle is counted from the day menstruation starts till the next onset of menstruation, i.e. after \(28\) days in the normal cycle. This cycle can be divided into \(4\) phases:

1. Menstrual phase
2. Follicular phase
3. Ovulatory phase
4. Luteal phase

1. Stages of Menstrual Cycle

1. This phase is characterised by the shedding of blood due to the breakdown of the inner uterine wall, i.e. endometrium.
2. This menstrual fluid consists of mucus, blood, cells of the endometrial lining, and unfertilised ovum. 
3. The first day of the bleeding is considered as day \(1\) of the menstrual cycle. This lasts for \(3-5\) days. This phase is called the menstrual phase.
4. Menstruation takes place when progesterone level is low in the body.
5. Normal menstrual bleeding is about \({\rm{40 – 50}}\,{\rm{ml,}}\) however, under various medical conditions, it can be up to \(125\,{\rm{ml}}\) also. This is one of the prime reasons why females are more prone to be anaemic.
6. Progesterone is the hormone that is secreted from the corpus luteum that maintains the thick lining of the endometrium.
7. Starting from the menstruation flow, the ovary starts to prepare a new egg in its follicle.

Did You Know?

The menstrual fluid never clots. This is due to the reason that the fluid contains fibrinolytic enzymes that break down fibrin protein that forms blood clots.

2. Follicular Phase

1. It is also called the preovulatory phase.
2. This phase is characterised by the proliferation of primary follicles to form Graafian follicles. 
3. It generally occurs from day \(6\) to day \(13\) of a normal menstrual cycle.
4. Low level of progesterone stimulates FSH, i.e. follicle-stimulating hormone in the ovary. 
5. Under the influence of FSH, many primary follicles in the ovary start maturing, but only one Graafian follicle develops. The other proliferated follicles degenerate, the reason why females reach menopause.
6. As soon as the Graafian follicle is formed, it starts producing estrogen hormone from theca interna layer.
7. Estrogen hormones provide negative feedback regulation to stop the synthesis and secretion of the FSH hormone. This prevents development of multiple Graafian follicles. Thus, only one ovum is released during each menstrual cycle.
8. Estrogen hormone is responsible for the regeneration of the endometrium layer which becomes \({\rm{2 – 3}}\,{\rm{mm}}\) thick. It also causes the development of secondary sexual characters in females, stimulates the release of LH hormone, and hence is the primary sex hormone of females.
9. This phase ends when ovulation takes place.

3. Ovulatory Phase

1. High estrogen will stimulate the pituitary gland to decrease the secretion of FSH and increase the secretion of LH, i.e. luteinizing hormone.
2. A high level of LH occurs between \(12-14\) days of the menstrual cycle and is called LH surge.
3. This LH is responsible for ovulation, i.e. release of a secondary oocyte from the Graafian follicle. 
4. This oocyte is arrested in the metaphase stage of meiosis II until fertilization.
5. There is an increase in body temperature after ovulation due to a high level of sex hormones.
6. After ovulation, the egg will move to the Fallopian tube and wait for sperm to fertilize. 
7. In the absence of fertilization, the egg will degrade after \(1-2\) days of ovulation. 
8. The day of ovulation in a menstrual cycle can be calculated by the formula \(\left( {{\rm{n – 14}}} \right),\) where \({\rm{n = }}\) number of days of the menstrual cycle.

4. Luteal Phase

1. It is also called the secretory phase or post-ovulatory phase.
2. In a normal menstrual cycle, it is a period between \(15-28\) days.
3. Ruptured Graafian follicle changes to the corpus luteum under the influence of LH.
4. This corpus luteum secretes progesterone hormone mainly and a small amount of estrogen.
5. This progesterone is responsible for further thickening (upto \({\rm{5}}\,{\rm{mm}}\)) of the endometrial lining. Also, it carries out vascularization of the endometrial lining.
6. Progesterone hormone will remain high throughout the gestation or pregnancy period if fertilization takes place. Thus, progesterone hormone is responsible for maintaining pregnancy.
7. Progesterone carries out negative feedback inhibition of GnRH hormone, thus preventing multiple ovulation.
8. As this phase is taking place under the influence of progesterone hormone, which is secreted by the corpus luteum, this phase is called the luteal phase.
9. If fertilization does not take place, then this corpus luteum regresses to corpus albicans which reduces progesterone level and the onset of the next menstrual cycle.

Table showing a summary of phases of the menstrual cycle.

Diagramatic Representation of Menstrual Cycle

Hormonal Changes During Menstrual Cycle

Different levels of hormones during the menstrual cycle direct different phases of the menstrual cycle. These hormones are described below:

1. The anterior pituitary is stimulated by the low level of progesterone during menstruation to secrete FSH to stimulate the ovary for follicular development and is seen during the follicular or preovulatory phase of the menstrual cycle.
2. Development of follicles in the ovary leads to the secretion of estrogen from the mature Graafian follicle, which helps in the formation of the endometrium, negative feedback inhibition of FSH, and development of secondary sexual characters in females.
3. High estrogen stimulates the anterior pituitary to decrease the secretion of FSH and increase the secretion of LH.
4. LH hormone is responsible for ovulation.
5. Progesterone hormone is secreted during the luteal phase by the corpus luteum. This hormone is responsible for further thickening and vascularization of the endometrial lining.
6. Progesterone also provides negative feedback inhibition to the GnRH hormone to stop the secretion of multiple ovums.

Reasons for Irregular Menstrual Cycle

1. The menstrual cycle is said to be irregular if menstruation comes before \(24\) days or after \(35\) days of the menstrual cycle.
2. If menstruation lasts for more than 8 days, it is said to be abnormal.
3. Bleeding occurring in a child or woman after \(50\) years of age is an abnormal condition. This irregular menstrual cycle is due to:
   a. Stress: This interferes with the brain that controls the menstrual cycle hormone. Disturbance in the menstrual cycle hormone results in an irregular menstrual cycle.
   b. Extreme weight loss: Less body calories interferes with the hormonal production which is needed for ovulation.
   c. Excessive exercise: Slight exercise is always fruitful and good for health. While excessive heavy exercise interferes with hormonal secretion, which may result in missed or stopped periods (amenorrhea).
   d. PCOS (Polycystic ovary syndrome): This results in missed periods, weight gain, infertility, excessive facial hair.
   e. Birth control pills: These pills may imbalance the menstrual hormones, which result in missed periods or periods with heavy bleeding or painful bleeding (dysmenorrhea).
   f. Uterus adenoma: Abnormal enlargement of the uterus or during uterine cancer.
   g. Hypothyroidism: Low level of thyroxine in the body can cause irregular menstrual cycles in females.

Summary

When a girl child is born, her ovaries already contain thousands of immature ova. When they attain puberty, one egg matures and is ovulated from the ovary every month. Uterus prepares itself to have the fertilized mature egg implanted in its bed.

Frequently Asked Questions (FAQs) on Menstrual Cycle

The most commonly asked questions about menstrual cycle are answered here:

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