Menstrual Cycle.



  • The menstrual cycle is the process of discharge of blood and broken tissues from the vagina of a woman every month from puberty to menopause except during pregnancy and some months of breastfeeding

  • It is a periodical process that produces changes in the female reproductive system which is responsible for the pregnancy.

  • The first onset of the menstruation cycle is called “Menarche”.

  • The permanent stoppage of the menstruation cycle is called “Menopause”.

  • Various hormones play important roles during the 28 days cycle.

  • For easy understanding the menstruation cycle is divided into two,

    • Ovarian cycle.

    • Uterine cycle.

Hormones in the Menstrual Cycle.

  • The hypothalamus releases a hormone called “Gonadotropin Releasing Hormone (GnRH)”.

  • The Gonadotropin Releasing Hormone acts on the anterior lobe of the pituitary gland to secrete two gonadotropins as,

    • Follicle Stimulating Hormone (FSH) (Required for  the maturation of ovarian follicles and also stimulates the release of estrogen.)

    • Luteinizing Hormone (LH) (It triggers ovulation).

  1. Ovarian Cycle

  • This involves the formation and maturation of follicular cells in the ovary

  • This cycle is divided into three phases as:

    1. Follicular phase.

    2. Ovulation phase.

    3. Luteal Phase.

  1. Follicular phase:

  • After onset of puberty, due to the release of a FSH and LH by the anterior pituitary, the ovarian follicles in the ovaries start to grow.

  • During the first stage, out of millions of follicular cells some selected cells start to grow. 

  • After a few days, the level of FSH increases which accelerates the growth of 6-10 primary follicles that compete for growth.

  • The second layer of cells called theca develops around these follicles which can produce other sex hormones like estrogen and progesterone.

  • LH from the pituitary and the estrogen within the follicles accelerates growth of the primary follicles into vesicular follicles.

  • After a week only one ovum starts growing, the other stops the growth, this is the stage which can determine the possibility of fraternal twins.

  • The single follicle further increases in size and forms the mature graafian follicle.

  1. Ovulation phase:

  • On 14 day of the 28 day cycle the LH surge happens which results in bursting of the graafian follicle inside the ovary resulting in the release of the ovum in the pelvic cavity.

  • Once released the fimbriae collect the ovum in the fallopian tube.

  • Ovum waits 24 hr for sperms, if sperms are available fertilization takes place or ovum dies.

  1. Luteal Phase:

  • Last stage of ovarian cycle.

  • The remaining part after ovulation forms a yellow colored body called “Corpus luteum”.

  • Corpus luteum starts producing the progesterone which inhibits release of the FSH and LH from the pituitary gland.

  • The increased progesterone blood levels cause proliferation of the endometrium, favoring the implantation of the embryo.

  • If the fertilization happens then the embryo releases a hormone called Human chorionic Gonadotropin Hormone (HcG); the HCG stimulates progesterone secretion from corpus luteum.

  • Progesterone is essential for maintenance of the pregnancy.

  • In absence of the HcG the corpus luteum disintegrates into corpus albicans.

  • Th decreased levels of progesterone cause breakdown of the endometrium.

2. Uterine Cycle.

  • The uterine cycle includes the changes in the endothelial layer of the uterus

  • It is divided into three phases as:

    1. Menstrual Phase.

    2. Proliferative Phase.

    3. Secretory Phase.

  1. Menstrual Phase:

  • Menstruation, also called menses, menstrual bleeding, or a period, is the first phase of the uterine cycle and indicates absence of the pregnancy.

  • Usually lasts about 3-5 days but might range from 2-7 days in some women. On average, 35 milliliters of blood is lost during menstruation, but 10ml to 80ml is considered normal.

  • In absence of the HcG the corpus luteum disintegrates into corpus albicans.

  • Th decreased levels of progesterone cause breakdown of the endometrium.

  • The release of prostaglandins cause contraction of the arteries leading to ischemia causing breakdown of stratum functionalis of endometrium.

  • The enzyme plasmin prevents clotting of blood during menstruation.

  • Pain in the back, stomach, and upper thigh is common during the first few days of menstruation and severe pain is commonly observed in adolescent girls (67% of women observe severe cramping).

  1. Proliferative Phase:

  • This is the second phase of uterine cycles where the estrogen causes the proliferation of the endometrial layer in the uterus.

  • After the maturation of follicles in the ovary, they cause the release of estrogen which causes the growth of a new layer of endometrium called proliferative endometrium

  • The endometrium becomes thick with the rapid cell multiplication and increases the mucus-producing cells as well as blood capillaries.

  • The estrogen also causes the formation of crypts in the cervix that facilitate the secretion of vaginal discharge.

  • This stage prepares the uterus for the entry of sperms.

  • This phase ends when ovulation occurs and the level of estrogen declines.

  1. Secretory Phase:

  • The final phase of the uterine cycle corresponds with the luteal phase of the ovarian cycle, which occurs after ovulation.

  • The corpus luteum releases progesterone hormone, which is particularly essential to make the uterus receptive for the implantation of the fertilized ovum

  • The endometrium becomes oedematous and the secretory glands produce a large amount of watery liquid to assist the passage of spermatozoa.

  • If fertilization occurs, the fertilized ovum travels to the uterus through the uterine tube and becomes embedded.

  • However, if fertilization doesn’t occur, menstruation occurs and a new cycle is initiated.

Commonly Asked Questions.

  1. Write a short note on the Menstrual Cycle.

  2. Describe different stages of the menstrual cycle.

Topics u may like to read