By BSc, MSc (embryologist).
Last Update: 03/24/2016

Menstruation is a process that is also known as period or monthly. Nonetheless, all of them make reference to the menstrual bleeding women experience monthly when fertilization has not taken place throughout their menstrual cycle, which leads to the breakdown of human endometrium.

The different sections of this article have been assembled into the following table of contents.

Why does it happen in girls?

It is s physiological, cyclical process that takes place in the last stage of the menstrual cycle. Once the woman’s body “detects” the egg has not been fertilized, the inner layers of the uterus—which had thickened to facilitate embryo implantation—are shed in the form of menstrual flow.

Menstruation may vary from cycle to cycle and each woman may present it in a different way. Menstrual flow contains vaginal discharge, blood, and endometrial tissue. It usually lasts from 3 to 5 days and menstrual blood loss is 40-50 ml approximately.

What is the purpose of having periods?

A girl’s first period usually starts during puberty, between ages 12 and 14, although some women may start having periods sooner or later. Throughout the first year, the menstrual cycle is likely to be irregular, since pubertal changes make the woman’s body experience several hormonal fluctuations. Thus, it is not uncommon that ovarian response is not a steady one at this stage.

From the very moment the body reaches sexual maturation, the woman’s organism prepares monthly for conception through the menstrual cycle. It should be borne in mind that, women do not produce eggs throughout their reproductive life. It is the opposite process to that experienced by men, who are continuously producing sperm in their lifetime. When they are born, women have a certain number of immature eggs in their ovaries that will start to ripen with the beginning of their period.

Cyclic recruitment of ovarian follicles takes place at the beginning of the menstrual cycle. Follicles are cyst-like structures containing eggs. Thanks to the stimulation triggered by hormones, ovarian follicle development and growth takes place in the ovaries. In the meantime, the lining of the womb is becoming thicker. This is what is known as endometrium, which function is to prepare the womb for embryo implantation and subsequent pregnancy.

Generally, out of all the follicles that develop in a single cycle, one of them grows to a larger extent, which will be the one releasing the egg. If fertilization by sperm has not taken place after the release of an egg, no embryo able to implant to the endometrium exists. Thus, the endometrial tissue is shed in the form of menstruation, thereby giving rise to the beginning of a new menstrual cycle.

Phases of the menstrual cycle

A woman’s body goes through two main stages or cycles: on the one hand, the ovarian cycle (i.e. oocyte development) and, on the other, the uterine cycle (i.e. endometrial growth).

As for the former, that is, the ovarian cycle, we can distinguish between the following phases according to follicle growth:

  • Follicular phase: At this point, the follicle goes through different stages, switching from primordial follicle to antral follicle or Graafian follicle, this one containing a mature egg before ovulation.
  • Ovulation phase: It refers to the release of a mature egg from the Graafian follicle, which is known as ovulation.
  • Luteal phase: When the follicle breaks down and releases an egg, the corpus luteum develops in the ovary, which is responsible for releasing estrogens and progesterone to trigger endometrial preparation and thickening.

If we pay attention to the endometrium, the uterine cycle can be divided into the following stages:

  • Proliferative phase: it takes place around days 5 and 13 of the menstrual cycle.
  • Secretory phase: it occurs between days 14 and 29 of the cycle.
  • Menstrual phase: this is what is called menstruation, which takes place around the first and fourth day of the cycle.

These two main phases of the cycle are linked to each other and work hand by hand as a consequence of hormone release both at the levels of the hypothalamic–pituitary–adrenal axis (HPA axis) and the ovaries.

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Authors and contributors

 Cristina Mestre Ferrer
Cristina Mestre Ferrer
BSc, MSc
Bachelor's Degree in Biological Sciences, Genetics & Human Reproduction from the University of Valencia (UV). Master's Degree in Biotechnology of Human Assisted Reproduction from the UV and the Valencian Infertility Institute (IVI). Embryologist at IVI Barcelona. More information about Cristina Mestre Ferrer

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    1. Sally Larson


      Pls, give me a piece of advice. My daughter had her first period 3 years ago now… The first year it was very irregular, but we didn’t pay attention at all because we knew first periods are never regular as you have just begun (when I was her age, the same happened to me). But it’s been 3 years now and it has become worse. She is 15 now and has periods like twice or three times in a year, just that. She doesn’t have normal periods like any other girl her age. I try to tell her it is common because her period came rather late (at age 12). What do you think?

      Pls, tell us something, I’m praying to God she’ll have normal periods soon!!

      • Hello Sally,

        If her periods started 3 years ago and they are still that irregular, that may be due to some hormone alteration or even polycystic ovaries.

        My advice is that you take her to a gynecologist in order to have a check-up done and find what’s the cause behind this.