Menstrual Cycle: Definition, Phases and Hormonal Changes

This article describes the phases and hormonal changes that take place in a fertile woman during the menstrual cycle, its length and variations. It also tells you when a woman is most fertile and likely to conceive during the cycle.

Definition and meaning

The menstrual cycle is defined as a recurring monthly cycle in which physiological changes take place in the woman’s and other female primates’ reproductive system with the production and midcycle release of a mature egg (ovum) from the follicle of the ovary and changes in the lining of the uterus (endometrium) that facilitate the implantation of the fertilized egg on the uterine wall.

The release of the egg is called ovulation. The five days preceding ovulation and the ovulation day are the most fertile days of the women during the menstrual cycle. Conception can occur only on these days if the sperm comes in contact with the egg.

Ovulation and the menstrual cycle occur every month during the reproductive years of the woman’s life. They function to prepare the woman for pregnancy every month and take a break only when conception occurs.

The cycle is instrumental to pregnancy and reproduction. Once conception occurs, no ovulation and menstrual bleeding occur during the full length of pregnancy.

The menstrual cycle starts appearing at puberty at an average age of 12 to 13 years and ends at menopause at an average age 52 years.

However, variations do exist and the first monthly bleeding is known to occur at ages between 8 to 15 years. Usually, the girl’s first period starts about 2 years after her breasts first begin to develop

Variations in the onset of menopause also exist and it can set in between the ages of 45 to 55 years.

Length of the menstrual cycle

You may be lucky and have regular menstrual cycles of regular length every month. But, variations do occur and that does not mean they are abnormal.

How long the menstrual cycle is, does vary in different women and also at times in the same woman.

The length of the menstrual cycle starts on the day your menstrual period begins and lasts till the onset of the next period. A typical cycle usually lasts for 28 days with ovulation taking place on the 14th day.

During the first few years after menstruation begins during your teenage years, long cycles are common. Later, menstrual cycles tend to shorten and become more regular as you age.

The cycles can last anywhere from 21 to 35 days in adult women and from 21 to 45 days in young teenage girls.

Use of certain contraception methods such as extended-cycle combined hormonal birth control pills (Seasonale) will alter the length of your menstrual cycle.

The physiological changes that take place during the menstrual cycle are under the control of the endocrine system and the physiological changes occur due to the changes in the hormonal levels preceding ovulation and after.

The menstrual cycle is divided into four phases/stages each with distinct characteristics and functions.

  1. Menstrual stage
  2. Follicular
  3. Ovulation
  4. Luteal

What is the menstrual phase?

The menstrual phase consists of menstruation or the woman’s monthly period and is the normal monthly bleeding from the vagina. It consists of the withered away unfertilized egg, the uterine wall lining (endometrium) and blood.

It occurs when no conception takes place during the previous cycle. Menstruation is triggered by the low levels of estrogen and progesterone during the luteal phase of the previous cycle.

During menstruation, the endometrium detaches from the uterine wall passes out of the body through the cervix and the vagina.

It lasts anywhere between 3 to 5 days. The first day of your bleeding is the first day of your menstrual cycle.

It is not necessary that all women have regular and smooth periods. Irregular periods are also commonly seen. Abnormalities in menstruation are known to occur and include:

  • Excessive bleeding (menorrhagia)
  • Absence of  period (amenorrhea)
  • Severe pelvic pain accompanying the bleeding or painful periods  (dysmenorrhea)
  • A prolonged period lasting 10 days or more
  • A scanty period where the duration of the bleeding is less than two days or the quantity of blood flow is less than 80 ml.
  • Fever

A normal menstrual cycle lasts 28 days. It could be seven days shorter or longer — 21 days or 35 days

An irregular menstrual bleeding is one if it occurs more often than every 21 days or lasts longer than 8 days.  A missed period, an early period, or a late period are all examples of an irregular menstrual cycle.

The phase preceding ovulation is called the follicular phase and the phase after is called the luteal phase.

Follicular Phase

The second phase is called the follicular or the proliferative phase. It begins on the first day after the menstrual period has ended and the levels of estrogen and progesterone hormones are the lowest during this time.

During this phase, the endometrium (inner lining of the uterus), which has been shed off during menstruation begins to regrow or proliferate after the menstrual bleeding and thickens in preparation for an implantation of an embryo should fertilization occur in the following cycle.

During the follicular phase, a few ovarian follicles are stimulated by the rise in the levels of the follicle stimulating hormone (FSH), which is secreted by the anterior pituitary gland (situated in the brain) during the end of the menstrual phase.

The development of follicles triggers the release of the estrogen hormone, which activates the uterine lining to thicken and facilitate the possible implantation of a fertilized egg.

Only one ovarian follicle reaches maturity. It is this matured follicle, which releases the ovum or egg into the fallopian tube during ovulation. Either of the ovaries could release the ovum and there is no fixed alternating pattern.

This phase lasts from the beginning of the menstrual cycle till the date of ovulation, which is normally 14 days but can vary anywhere between 10 to 14 days.

Ovulation phase

Ovulation in the menstrual cycle is defined as the release of a mature egg or ovum from the follicle of one of the ovaries into the fallopian tube.

The ovaries produce estrogen and progesterone hormones whose levels begin to rise and trigger the release of the gonadotropin-releasing hormone, which in turn triggers a surge in the levels of the luteinizing hormone (LH).

It is the LH surge that causes the release of the ovum from the ovarian follicle.

The life of the ovum is only 12 to 24 hours and therefore, this phase lasts for only one day. Your fertility is at its peak during ovulation.

For details, read the full article on ovulation. You could also read symptoms that tell you are ovulating

Luteal phase: After ovulation and before period

The luteal phase, also called the secretory phase, begins the day after the ovulation day until the first day of the next menstrual period. This is the phase of the corpus luteum, hence the name “luteal phase”.

If there is no conception within 48 hours after ovulation, the egg moves down the fallopian tube, disintegrates and is absorbed into the uterine wall.

The follicular theca cells luteinize into small luteal cells and the follicular granulosa cells luteinize into large luteal cells forming the corpus luteum. Luteinize is the process of forming the corpus luteum. FSH and LH hormones both secreted by the anterior pituitary gland bring about this conversion of the follicle into the corpus luteum after ovulation.

The levels of estrogen and progesterone hormones, particularly progesterone, rise during this phase to prepare the female reproductive tract and body for pregnancy.

These hormones are secreted by the follicle from which the ovum was released and the follicle now has become the corpus luteum.

The uterus inner lining called the endometrium undergoes changes and becomes thick due to these hormonal changes to prepare for implantation of the fertilized ovum and pregnancy.

If conception does not occur, the pituitary gland reduces the production of LH and FSH whose levels begin to fall. Consequently, the corpus luteum begins to atrophy and withers away.

As a result, estrogen and progesterone levels begin to fall and this triggers the endometrium to shed off during the next menstruation period along with the unfertilized ovum.

Changes in duration that occur in the length of the menstrual cycle are due to variations in the proliferative phase. The luteal phase usually stays constant and mostly lasts for 14 days but can vary anywhere between 12 to 16 days. The length of the luteal phase must be at least 10 days to maintain pregnancy.

 

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