The menstrual cycle is a very important part of women’s life and can affect them in various ways. Understanding the menstrual cycle phases and the regulating hormones, helps a woman know her body better. This is what we will try to accomplish in this article.

First of all, what is the menstrual cycle?

According to studies, girls typically have their first period between the age of 9 and 15, although deviations aren’t unusual. For 3-7 days every month, women experience their period in various degrees of intensity. Nevertheless, the menstrual cycle consists of 3 phases with the period itself being only the first one. The average menstrual cycle length is 28 days.

What happens during the menstrual cycle phases and which hormones regulate them?

To begin with, let’s talk about the major organs participating in this process. Like most functions of the human body, the menstrual cycle is controlled by the brain. Two of the most important brain parts responsible for hormone production are the hypothalamus and the pituitary glands. Of course, the menstrual cycle phases take place in the female genital organs with the ovaries, the fallopian tube and the uterus playing a key role. In every woman’s ovaries, there are hundreds of thousands of follicles that are aggregations of cells. Every follicle contains one immature ovum (egg).

1st-10th day of the cycle

The first day of the menstruation, easily noticed by every woman, is also the first day of the menstrual cycle. What women don’t notice though is the chain of events that start in the brain. When the menstruation begins, the hypothalamus of the female brain starts producing large amounts of a hormone called gonadotropin (GnRH). This hormone is immediately transferred to the pituitary glands which in turn release 2 other menstrual cycle hormones :

the follicle stimulating hormone (FSH)

and the luteinizing hormone (LH)

An amount of FSH moves to the ovaries where it stimulates the maturation of some of the follicles. These follicles grow in size and start releasing small amounts of a group of hormones, called estrogens. In small concentration, estrogens inhibit the production of LH in the pituitary glands and at the same time they increase the thickness of the inner membrane of the uterus, also known as the endometrium. The reason for that is that the endometrium needs to be ready for a potential implantation of the fertilized egg.

10th – 14th day of the cycle

After about 10 days begins the second menstrual phase. Then, the amount of estrogens is high enough in order to cause the sudden and rapid increase of LH production in the pituitary glands. High levels of LH are capable of provoking the ovulation, meaning the rupture of the largest of the follicles that were growing. The mature ovum of that follicle can now enter the fallopian tube and start its journey to the uterus. Every ovum can survive nearly 12-24 hours, during which it can be fertilized by a spermatozoon.

Right after the ovulation, the empty follicle turns into a structure called corpus luteum and starts shrinking. Meanwhile, it produces estrogens and another very important hormone, the progesterone.

15th-28th day of the cycle

Approximately, on day 21 of the menstrual cycle, progesterone concentration is high enough to cause:

the reduction of the GnRH production and consequently of the FSH and LH production in order to prevent the maturation of other follicles

significant growth of the endometrium thickness for a potential implantation of the fertilized egg

In case no fertilization occurs, the corpus luteum normally degenerates, the production of progesterone stops and as a result, the thick inner layer of the uterus starts to collapse and is eventually being shed by the female body (menstruation). The production of GnRH is once again possible and a new menstrual cycle can begin.

In case the ovum is fertilized, the zygote is implanted in the endometrium after about 6 days of its formation. There, it promotes the production of another significant hormone, called human chorionic gonadotropin (hcG). HcG keeps the corpus luteum active in order to continue the production of estrogen and progesterone and maintain the thick endometrium which is necessary for the proper development of the embryo.

Summary

As we can see, there are a lot of hormonal changes in menstrual cycle. Aside from their contribution in the menstrual cycle phases, some of these hormones are also responsible for the premenstrual syndrome (PMS). Typical PMS symptoms are period bleeding, pain, irritability, swelling and mood changes. Despite all its unpleasant consequences, the menstrual cycle is an essential function of the female body as it can lead to the creation of a new life!

Sources

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