Virtual Medical Centre

Menstruation (Period or Menstrual Cycle)

• What is menstruation?
  • Menstruation calculator
• The hypothalamus
  • GnRH

What is menstruation? 

Menstruation describes the female period. The menstruation cycle involves the monthly release of an egg (ovum) in a process called ovulation, with bleeding due to shedding of the uterine lining at the end of the cycle. Normal menstruation is a highly complex interactions between a number of hormones produced by three organs of the body; the hypothalamus; the pituitary gland and the ovaries. The interactions between these organs are referred to as the hypothalamic-pituitary-ovarian axis (HPO axis). The cycle has an average duration of 28 days, but the normal range is between 21 to 35 days. Menstruation usually starts at an average age of 13 (called menarche) and lasts on average till age 51 (called menopause).

The inner lining of the uterus (the endometrium) goes through three phases during the menstrual cycle: the proliferative phase, the secretory phase and the menstrual phase. The changes in this lining is needed for provision of an optimal environment for the fertilised egg to grow, which is why is needs to be shed and renewed monthly. The ovarian cycle is divided into two phases: the follicular phase and the luteal phase, during which different levels of hormones are released. Bleeding as a result of menstruation lasts 2 to 7 days on average, from which there is an average loss of 20 to 80mL of blood.   

Menstruation Calculator - Calculate when your next period is due.

The hypothalamus

There are five different hormones released from the hypothalamus that have an effect on the menstrual cycle. These hormones include:

• Gonadotrophin releasing hormone (GnRH)
• Thyrotropin releasing hormone (TRH)
• Somatotropin release-inhibitory factor (SRIF) aka Somatostatin
• Corticotrophin releasing factor (CRF)
• Prolactin release-inhibiting factor (PIF)

Each of these have a different effect on the anterior pituitary gland, stimulating it to release or stop releasing a particular hormone.

Gonadotrophin releasing hormone

Release of this hormone is responsible for the stimulation of specific cells called gonadotrophs in the pituitary gland. This stimulation results in the production of two important hormones called luteinising hormone (LH) and follicular stimulating hormone (FSH) from the pituitary. GnRH is of great importance in the menstrual cycle. One of the most important features of GnRH release is the fact that its release occurs in a pulsatile fashion. At the start of puberty there is a marked increase in the frequency and amplitude of GnRH release.

A part of the brain called the surge centre controls the timing of this increased release of GnRH. The surge centre is present in females very early in life, however it is only as puberty approaches that this centre becomes more responsive to hormonal changes. Throughout the menstrual cycle there is pulsatile release of GnRH. Anything that interferes with the pulse frequency of GnRH can stop the menstrual cycle from occurring. Restoration of this pulsatile GnRH by administering hormones can produce a return to ovulation.