The stages of the mentrual cycle
Changes during the Mentrual Cycle

Two important hormones govern the menstrual cycle:

  • estrogen
  • progesterone

Hormones are chemical messengers, which the body uses to send instructions from one part of the body to another. The levels of estrogen and progesterone signal the changes that happen during the menstrual cycle. It is important to remember that these hormones also influence other parts of the body. For example, estrogen helps a woman retain calcium in her bones. The influence of these hormones is also thought to cause many of the symptoms of premenstrual syndrome.

A woman’s menstrual cycle is said to begin on the first day of her bleeding. A woman’s menstrual cycle only occurs if she is not pregnant.

Day 1

  • estrogen and progesterone levels are at their lowest level
  • the inner lining of the uterus, or endometrium is discharged as menstrual blood
  • the unfertilized ovum produced in the last cycle is also discharged

Day 2-12

  • menstruation continues for three to six days for most women
  • when menstruation begins, a new ovum begins to mature in the ovaries
  • the sac around the maturing ovum produces estrogen, increasing the levels in the body
  • increasing estrogen levels prompt the uterine lining to thicken beginning around day nine
    If a woman becomes pregnant this nutrient-rich lining supports the developing embryo.


  • estrogen levels peek
  • around Day 14 the sac containing the mature ovum, splits open releasing it from the ovary
    This is called ovulation. Some women feel a slight pain when this occurs. This is called a mittelschmerz. Some women also have spotting (light bleeding) at this time.
  • the endometrium continues to thicken

Days 15-22

  • the empty sac left in the ovary begins to produce both estrogen and progesterone
    This sac is called the corpus leuteum.
  • the uterine lining continues to thicken thanks to estrogen produced in the ovary
  • the ovum travels from the ovary down the fallopian tube
    If the egg is going to be fertilized (unite with a sperm) it is likely to happen now. When a fertilized egg reaches the uterus, high levels of estrogen and progesterone signal the uterine lining to allow it to implant on the wall of the uterus.

Day 22 – Day 1 of next cycle

  • around this time the corpus luteum stops producing estrogen and progesterone
    If the egg has not been fertilized, levels of both estrogen and progesterone will begin to drop.
  • blood vessels in the uterine wall contract and spasm due to the lack of estrogen and progesterone
  • the uterine lining is shed as menstrual blood beginning the first day of the new cycle