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The Menstrual Cycle

What Happens During Your Menstrual Cycle


Updated May 30, 2014

The timing and amount of blood flow you experience during your monthly menstrual cycle depends on the coordinated performance of your endocrine glands, which produce the hormones necessary for menstruation to occur when pregnancy does not. What they do affects what happens in your reproductive organs.

First, What Are the Reproductive Organs?

The uterus is a pear-shaped organ which, in its non-pregnant state, is collapsed and about the size of your fist. It is located between the bladder and the lower intestines.

The lower third of the uterus is called the cervix. The cervix has an opening called the "os," which opens into the vaginal canal and permits your period to flow out.

Extending from each side of the uterus are the fallopian tubes. 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. These follicles contain the material necessary to produce eggs.

The inner lining of the uterus is called the endometrium. The endometrium sheds during menstruation. In addition to endometrial tissue, your menstrual flow also contains blood and mucus from the cervix and vagina. When pregnancy occurs, the endometrium thickens and fills with blood vessels that mature into the placenta that contains the growing fetus.

Which Hormones Interact with the Reproductive Organs?

The area of the brain called the hypothalamus, together with the pituitary gland, which also is in the brain, control the hormones necessary for reproductive health.

Six hormones serve as chemical messengers to your reproductive system. These hormones include:

  • Gonadotropin-releasing hormone (GnRH)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Estrogen
  • Progesterone
  • Testosterone

During your menstrual cycle, GnRH is released first by the hypothalamus. This causes a chemical reaction in the pituitary gland and stimulates the production of FSH and LH. Estrogen, progesterone, and testosterone (yes, the "male" hormone) are produced by the ovaries in reaction to stimulation by FSH and LH. When these hormones work harmoniously, normal menstrual cycles occur.

Your Menstrual Cycle in Phases

The menstrual cycle is divided into two phases--the follicular or proliferative phase; and the luteal or ovulatory phase. The follicular phase includes the time when menstruation occurs and is followed by proliferation or the growth and thickening of the endometrium. This phase typically lasts from 10 to 14 days, starting with the first day of menstruation.

Estrogen and progesterone levels are at their lowest during menstruation. When bleeding stops, the proliferative phase begins causing the endometrium to grow and thicken in preparation for pregnancy. During the next (approximately) two weeks, FSH levels rise causing maturation of several ovarian follicles and the size of the eggs triple.

FSH also signals the ovaries to begin producing estrogen which stimulates LH levels to surge at around day 14 of your cycle triggering one of the follicles to burst, and the largest egg is released into one of the fallopian tubes.

This phase is followed by the premenstrual phase, known as the luteal phase. This premenstrual period lasts approximately 14 days. After ovulation, LH causes the corpus leuteum to develop from the ruptured follicle. The corpus leuteum produces progesterone.

Together estrogen and progesterone stimulate the endometrium to prepare a thick blanket of blood vessels that will support a fertilized egg should pregnancy occur. When pregnancy occurs, this blanket of blood vessels becomes the placenta which surrounds the fetus until birth.

When pregnancy does not occur, the corpus leuteum deteriorates and becomes the corpus albicans. Once this occurs, progesterone and estrogen levels decline, and the endometrial lining is shed during menstruation.

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