Menstrual periods from start to finish

The menstrual cycle involves several events in females and is controlled by hormones. These changes help to prepare the body for a potential pregnancy each month. During a period or menstruation the uterus sheds the thickened endometrium and the blood flows from the uterus and passes out of the body through the vagina.

The menstrual flow is made up of tissue, cells, blood and mucous. The total amount of each period is about 60 - 80 millilitres or half a cup. Most periods last from 3 to 5 days. Periods may be heaviest or lightest at the beginning of the period and the colour can range between black, brown, dark red, bright red and pink.

A menstrual cycle starts on the first day of the period (referred to as day 1) and ends the day before the next period starts. The average menstrual cycle is 28 days but cycles can range anywhere from 21 to 35 days in adults and 21 to 45 days in young teenagers. For the first few years after menstruation begins it is common for the cycles to be longer and irregular.

After the period, rising levels of the hormone help thicken the lining of the endometrium and approximately half way through a cycle, an ovum is released from one of the ovaries (this is called ovulation). If the ovum is fertilised as it travels down the fallopian tube, it lodges in the endometrium and a pregnancy may occur.

If the ovum is unfertilised the endometrium will break down and the blood and tissue will pass through the vagina as a period.

The start of the periods is called menarche. A girl usually starts her periods between the ages of 11 and 13, although it may be earlier or later and still be normal. The age when a girl starts her period is influenced by biology and genetic and environmental factors, especially nutrition. A female will continue to have periods until she is about 50 years of age. The last menstrual period is known as the menopause.

A female is born with the total number of ova (about one to two million immature ova or follicles) in her ovaries. By the time she reaches puberty, only about 400,000 follicles remain. When a girl reaches puberty one of the ova matures and is released each month. The ovaries also start to produce important hormones called oestrogen and testosterone. At the start and end of reproductive life the hormones fluctuate, ova are released less frequently and the menstrual cycle is often irregular.

The time of fluctuating hormones at the end of reproductive life is called the peri-menopause and can be associated with other symptoms such as hot flushes and night sweats that can be relieved by replacing the hormones. After menopause the oestrogen level stays at a low level.

From the age of 30 years, a woman ceases to release an ovum each month. This is reflected in the decreasing natural pregnancy rate with age which is quoted as ≤29 years – 14%; 30–34 years 12%; ; 35–39 years - 5%; >40 years – 3%

Even though the release of an ovum is infrequent after 45 years and rare after menopause, an isolated ovulation is still possible, so contraception is advised until a year after menopause when this occurs at aged 50, and for two years when menopause occurs earlier.

Reference: Fukuda M et al. Characteristics of human ovulation in natural cycles correlated with age and achievement of pregnancy. Hum. Reprod. (2001) 16 (12): 2501-2507