What Happens During Your Menstrual Cycle

What Happens During Your Menstrual Cycle

The cyclical changes that occur during the normal menstrual cycle in women
The cyclical changes that occur during the normal menstrual cycle in women. Getty Images/Encyclopaedia Britannica/UIG

During your menstrual cycle, your body's reproductive system undergoes physiological and hormonal changes.

To understand and discuss these changes, let's review some anatomical facts:

  • The uterus, or the womb, is a pear-shaped organ, about the size of your fist, located between your bladder and lower intestines.
  • The cervix is the lower third of the uterus and it's opening, called the "os," is the entrance into the vaginal canal and permits your period to flow out.
  • The fallopian tubes extend from each side of the uterus and near the end of each fallopian tube is an ovary.
  • The ovaries are almond-sized organs which produce eggs. Each ovary contains from 200,000 to 400,000 follicles, which contain the material necessary to produce eggs.
  • The endometrium is the inner lining of the uterus and this comes out as your menstrual flow. In addition to endometrial tissue, your menstrual flow also contains blood and mucus from the cervix and vagina. When you're pregnant, the endometrium thickens and fills with blood vessels that mature into the placenta.

Hormones involved in your menstrual cycle

It all starts with your endocrine glands because they produce the hormones that determine when you get your period, the amount of menstrual flow and what happens to your reproductive organs.

The area of the brain called the hypothalamus connects your nervous and endocrine system by way of the pituitary gland, which also is in the brain, and controls the hormones necessary for reproductive health and your period.

Six hormones serve as chemical messengers to your reproductive system:

  1. Gonadotropin-releasing hormone (GnRH)
  2. Follicle-stimulating hormone (FSH)
  3. Luteinizing hormone (LH)
  4. Estrogen
  5. Progesterone
  6. Testosterone

During your menstrual cycle, the hypothalamus first releases GnRH. This causes a chemical reaction in the pituitary gland and stimulates the production of FSH and LH.

Your ovaries produce estrogen, progesterone, and testosterone (yes, the "male" hormone)  in reaction to stimulation by FSH and LH. When these hormones work harmoniously, normal menstrual cycles occur.

Your Menstrual Cycle in 4 Phases

Keep in mind the menstrual cycle can vary greatly from woman to woman or month to month and still be considered normal. Generally, the length of your menstrual cycle can fluctuate from 3 weeks to 5 weeks, without alarm.

When counting the days in your cycle, always count the first day of your period as day one. The average period lasts about 6 days, although some women may experience slightly shorter or longer periods.

Your menstrual cycle happens in four phases:

  1. the menstrual phase
  2. the follicular phase
  3. the ovulation phase
  4. the luteal phase

The Menstrual Phase

The menstrual phase starts the moment you get your period and typically lasts up to five days. During this time, your uterus sheds its lining through your vagina and women wear a tampon or sanitary pad to absorb it.

The Follicular Phase

The follicular phase comes next and is usually during days six through 14 of your cycle. Your estrogen levels rise, causing the endometrium to get thicker. FSH levels also rise causing maturation of several ovarian follicles, one of which will produce a fully mature egg during days 10 to 14.

The Ovulation Phase

Around day 14, in a woman who has a 28-day cycle, LH levels surge causing ovulation. This means one of the mature follicles burst and released the fully mature egg into one of the fallopian tubes.

The Luteal Phase

The fourth stage, called the premenstrual or luteal phase, lasts approximately 14 days. At this time, the egg travels through the fallopian tube to the uterus. If it's fertilized by a sperm, you get pregnant. If not, progesterone and estrogen levels decline, and the endometrial lining flows out as your period.


American Congress of Obstitricians and Gynocologists: Menstruation

Cleveland Clinic: Menstrual Cycle (2015)

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