Also known as menses, menstrual period, period.

Menstruation, or a woman's period, is normal vaginal bleeding that occurs as part of a woman's monthly cycle.

Every month, a woman's body prepares for pregnancy. If no pregnancy occurs, the uterus, or womb, sheds its lining. The menstrual blood is partly blood and partly tissue from inside the uterus. It passes out of the body through the vagina.

As girls go through puberty, periods usually start between age 10 and 16 and continue until menopause.

As well as vaginal bleeding, you may also have one or more of the following:

  • abdominal or pelvic cramping
  • bloating and sore breasts
  • lower back pain
  • food cravings
  • mood swings and irritability
  • headache and fatigue.

Common features

The range of ‘normal’ for periods is wide, but they have some common features.

Start age (menarche)

  • Menstrual periods generally start between 10 and 16 years of age; around two years after the breasts first start to develop.
  • Girls often start their periods at a similar age to other women in their family - if you can, find out when your mother, sister or grandmother started.

Cycle pattern

  • Periods are often not regular when they first start, but generally settle into a pattern after about three years.
  • Some women never have a regular cycle.
  • Periods occur anything from every 21 to every 40 days.
  • The much-quoted cycle of 28 days occurs in just one woman in 10.
  • The first day of bleeding is counted as day 1 for a woman's cycle.
  • If someone has a 4-week cycle, they ovulate (release an egg) around day 14 and have a period 14 days later (if a pregnancy has not occurred).
  • If your cycle is 35 days long, then you are likely to ovulate around day 21.
  • Contraception is needed throughout your cycle as women have become pregnant at other times.


Blood loss

  • About 30-40ml of blood is lost per period, but it can look like more. To help gauge how much blood you lose, estimate that a tampon holds around 5ml of blood, while sanitary pads hold anything from 5-15 ml each.
  • Bleeding lasts around three to five days.
  • Bleeding is generally heaviest for the first day or two, lighter over the next day or two, maybe finishing with a day or so of spotting.

Blood clots

The blood does not usually clot, but small amounts of tissue from the lining of the uterus are normal and can look a little like clots. The difference is that clots are dark in colour and are generally accompanied by heavy bleeding, while uterine tissue is lighter coloured.

Level of discomfort

Some discomfort is normal, but periods are not generally painful.

Approaching menopause

  • As women approach menopause, periods may become more irregular again and tend to finish between the age of 40 and 55. Read more about menopause.
  • Periods with features differing from this may suggest an underlying problem.

What if my periods are not normal?

No periods

Only under the following circumstances is it normal to have no periods:

  • before you reach puberty
  • when you are pregnant
  • if you are breastfeeding
  • if you have reached menopause.

If you have no periods, but you should, it is known as amenorrhoea. There are two categories of amenorrhoea.

  • Primary amenorrhoeaYour periods have not yet started and you are 16 years or older.
  • Secondary amenorrhoeaYou usually have regular periods, but they have stopped for six months or longer.

Amenorrhoea may be caused by:

  • strenuous exercise
  • stress
  • some medications, including the oral contraceptive pill (it can take three to six months to start periods again after stopping the pill)
  • chronic illness
  • problems with the ovaries
  • hormonal imbalances such as polycystic ovarian disease, malfunction of the thyroid gland, hypothalamus (part of the brain that helps regulate the menstrual cycle) or pituitary gland
  • anatomical problems with the uterus, cervix or vagina
  • the absence of puberty
  • depression and some other forms of mental illness
  • low body weight.

Heavy periods

Menorrhagia is the term given to periods that are long and heavy. Menorrhagia affects 10-15% of women and is one of the main reasons for having a hysterectomy. It may involve periods that are prolonged, lasting for longer than seven days, and/or excessive bleeding with flooding or clots.

  • Women with menorrhagia lose 80ml or more of blood per period, compared with the 30-40ml lost typically. If you are changing your tampon or sanitary pad more often than every hour, flooding or becoming anaemic, this is a good indication you have menorrhagia.

  • Read more about heavy periods

Learn more

Menstruation Kids Health NZ 
All about Menstruation (For Teens) Puberty and Periods, Nemours Foundation and Kidshealth, 2015
Menstruation Medline Plus and National Institute of Child Health and Human Development (NIH), 2014
Menorrhagia in adolescents - Assessing Heavy Periods Medicine Today, 2009

Credits: Reviewed by Dr Martin Sowter, FRCOG FRNZACOG, 2009. Reviewed By: Health Navigator