Menstrual cycle (menstruation)

May 25, 2018

The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible.
It refers to the series of events that occur in the ovaries of sexually mature and non-pregnant females and culminate in menses.
The menstrual cycle is approximately 28 days long.
The cycle is required for the production of oocytes.
This cycle is also for the preparation of the uterus for pregnancy.
The beginning of this cycle marks the onset of puberty in human females.
It begins at the age of 12-14 years and goes till the age of 50-55 years.
During each menstrual cycle, an egg matures and is positioned to meet a sperm cell in the fallopian tube.
If the egg is not fertilized, it is then washed out.
Up to 80% of women report having some symptoms during the one to two weeks prior to menstruation.
Common symptoms include acne, tender breasts, bloating, feeling tired, irritability and mood changes.
The complete menstrual cycle involves four different phases. They are:

1. Follicular phase:

It is the period prior to ovulation.
It begins when the hypothalamus produces a releasing factor.
The releasing factor stimulates the anterior lobe of the pituitary gland to release follicle stimulating hormone (FSH).
It is transported through the blood stream and causes a group of ovarian cells, called a follicle to form the egg.
Simultaneously, FSH promotes the production of the hormone oestrogen, which is released in the blood stream.
Oestrogen, in turn, stimulates the lining of the uterus to thicken and the pituitary gland to produce luteinizing hormone (LH).
This hormone then causes the maturation of the eggs.

Image source: researchgate

2.Ovulation:

As the egg matures, the follicle moves towards the surface of the ovary, ruptures and releases the ripe eggs. This is called ovulation.
It happens during middle of the menstrual cycle.
During ovulation, the small finger-like projections (fimbriae) draw the egg from the ovary into the fallopian tube.
The egg remains in the fallopian tube for about 4 days and can be fertilized during this time.
The egg or zygote (if it is fertilized) then moves through the fallopian tube to the uterus, which is now thick and lined with mucus.

3.Luteal phase:

The luteal phase is the period between the ovulation and the onset of menstruation.
It lasts for 14 days, i.e. from 15^th to 28^th day of the cycle.
During this period, the ruptured follicle left on the surface of the ovary, develops into a golden coloured new structure, called the corpus luteum.
It secrets a hormone progesterone, that further prepares the uterus for the anticipated pregnancy.
The uterine lining becomes thicker and almost ready to nourish and protect the fertilized egg.
The secretion of progesterone and its level in blood is controlled by luteinizing hormone.

Image source: oviahealth

4.Menstruation:

If the egg was fertilized in the fallopian tube, the resulting embryo attaches to the lining of the uterus.
If the egg was not fertilized, it will not attach to the lining.
In this case, corpus luteum stops producing progesterone and the soft tissues of uterine lining are cast off.
These tissues along with the unfertilized egg are discharged in the last stage of the cycle.
This is called menstruation.
In other words, menstruation or menses is a period of mild bleeding (haemorrhage) during which the uterine epithelium is sloughed and expelled from the uterus.
It usually occurs every 28 days.
The bleeding lasts for about 4-5 days. At this time, the level of oestrogen and progesterone is very low in the blood.
Following the menstruation the cycle begins again.