The human menstrual cycle involves a complex and regular change in female anatomy and physiology over an approximate monthly time period which con consists of a number of bodily changes associated with the development of an egg and the possibility of pregnancy. The cycle starts with the first day of the menstrual period (referred to as day one) and ends the day before the next period starts. These cycles commence at puberty and cease at menopause.
While the length of the menstrual cycle is often 28 days, there is no real 'average' length, as menstrual cycles can vary anywhere from 20 days to 40 days. Cycles longer than six weeks are considered unusual.
The length of a woman's menstrual cycle may also change through different life stages. Irregularities are common during adolescence and in the time approaching menopause. Psychological factors such as stress and emotional distress, changes in weight, excessive physical activity and travelling can also cause irregularities in a woman's menstrual cycle.
Phases of the cycle
Conditions and Disorders of Menstruation
Click on the desired topic to get to that section
Menarchae or Menopause
Phases of menstrual cycle
There are four distinct phases of the menstrual cycle: menstruation, follicular phase, ovulation and the luteal phase. In order to properly understand menstruation it is necessary to first explain the phases that precede it.
The follicular phase is the phase from the first day of menstruation to ovulation. During this phase the follicle-stimulating hormone (FSH) is released by the pituitary gland, located in the brain. FSH causes between 10 and 20 follicles to begin developing in the ovary. Usually only one of these follicles will mature to become the ovum (egg), with the others degenerating. The developing follicles produce the female hormone oestrogen which causes the endometrium (lining of the uterus) to become thick in preparation for the possible embedding of a fertilised egg (1). The follicular phase can vary considerably in length, depending on the time of ovulation.
The rise in oestrogen during the follicular phase leads to the secretion of the gonadotropin-releasing hormone (GnRH) from the hypothalamus, located in the brain. This in turn increases the pituitary gland's production of both the luteinising hormone (LH) and FSH. The abrupt rise in the LH triggers ovulation, which is the release of the ovum (egg) from the ovary. Following ovulation the ovum is swept into the fallopian tube by the fimbria (fringelike extensions) and moved along towards the uterus. If fertilisation does not occur the egg will disintegrate over the next 12 to 24 hours (2).
This phase is the time from ovulation to the beginning of menstruation. During this phase the remnants of the follicle from which the egg was released become the corpus luteum. The corpus luteum secretes large amounts of the hormone progesterone and some oestrogen. While oestrogen allows the endometrium to thicken further, progesterone maintains the lining and also contributes to its development. If fertilisation does not occur the corpus luteum begins to degenerate and progesterone levels decline leading to the disintegration of the endometrium.
Women may experience other bodily changes during this phase, including tender or lumpy breasts, fluid retention and bloating. Mood swings, tiredness and feelings of anxiety, anger and sadness are also common at this time.
Menstruation occurs when the broken down endometrium flows out through the vagina. The menstrual fluid is actually made up of several components other than blood including endometrial cells, cervical mucus and vaginal secretions (3).