A woman’s menstrual cycle may be long, short, predictable or unpredictable. It may change from month to month and over the years, or it may stay pretty much the same. What is normal for one woman is not necessarily normal for another. But there are key changes in every woman’s cycle that can help you get to know what is normal for you. Whether you simply want to know more about your body, are trying to increase your chances of getting pregnant or want to avoid pregnancy, these information pages explain how the menstrual cycle works and what you can do to get to know yours better. 

Periods are a normal part of a woman’s life from around the age of 10-14 until about 50. This regular (or not so regular) bleeding is the most noticeable sign of a woman’s menstrual cycle, but it is not the only sign. Getting to know the other, less obvious signs of your cycle can help you become more familiar with your own changing levels of fertility. The length of a woman’s menstrual cycle is calculated by the number of days between one period and the next.

A cycle begins on the first day of bleeding and continues up to, but not including, the first day of the next period. Women’s cycles range from 21 to 40 days or more, with an average of around 28 days. The length of a woman’s cycle may change a little or a lot from month to month
Bleeding (menstruation) can last from 1 to 8 days, with the average being 4 to 5 days. The amount of blood a woman loses during her period tends to remain the same from one cycle to the next, but some women notice a change over time.

The menstrual cycle explained

When estrogen and progesterone levels in a woman’s body are at their lowest – just before her period – two things begin to happen:

1. The lining of the womb starts to shed. This is your period and is considered the beginning of a new cycle. If you are counting the days of your cycle, count the first day of bleeding as day one.

2. At about the same time, the brain’s pituitary gland starts releasing a hormone called FSH (follicle stimulating hormone). The FSH triggers follicles (eggs in their sacs) to start maturing in your ovaries.

Menstruation to ovulation

Under the influence of FSH, 10 - 20 follicles start to develop, but only one of these (sometimes two) will mature fully. As the follicles grow, they release increasing amounts of estrogen and this estrogen causes the lining of the uterus to thicken in preparation for a fertilised egg. This stage of a woman’s cycle begins at the same time as menstruation and can vary greatly in length. It may last anywhere between 6 and 21 days and is called the follicular or pre-ovulatory phase. Women in this phase are considered semi or partly fertile because there is no way of knowing how many days it will be until ovulation.

Ovulation

Once the estrogen produced by the growing follicles reaches a certain level, it triggers the pituitary gland to release a surge of LH (luteinising hormone). This causes the most mature follicle to burst open and release its egg into the Fallopian tube. This is ovulation.

Some women feel a slight twinge on one side of their lower back or abdominal area around the time of ovulation. Not every woman experiences this, but it is normal and is known as mittelschmerz (middle pain). Some women may also have discharge that is pinkish or a little bloody. If you have more than this little bit of bleeding between periods, tell your doctor.

The few days leading up to ovulation are considered the most fertile in a woman’s cycle. This is because sperm can survive for up to seven days in a woman’s body (on average they live for 3 days). If a woman has sex or insemination during the six or seven days before she ovulates, it is likely that the sperm will still be around by the time her egg is released.

One or two days after ovulation are also considered fertile days because a woman’s egg can live for about 20 hours after ovulation. If two eggs have matured, the second will be released within 24 hours of the first. From a few days after ovulation until her next bleed, a woman is generally not fertile.

Anovulatory cycles

Although ovulation occurs in most cycles, it is possible to have a cycle, and a period, without ovulating. This is called an anovulatory cycle and may happen in young women who have just started menstruating, women who are breastfeeding, women nearing the menopause, and women whose cycles are longer than 35 days.

Ovulation to menstruation

This phase is called the luteal or post-ovulatory phase. It is generally accepted that the time from ovulation to menstruation is always 12 to 16 days, whether your cycle is short, average or long. But while this phase does tend to be more constant than the phase before ovulation, recent research suggests it may range from 7 to 19 days.

After the egg has been released at ovulation, the empty follicle starts to produce progesterone as well as estrogen. The progesterone causes the lining of the womb to secrete nourishing fluids. If the egg becomes fertilized, it plants itself into the womb lining and the follicle it came from continues to produce progesterone to ‘feed’ the fertilized egg.

If fertilization does not occur, the follicle starts to break down and slowly stops producing hormones. When the follicle has broken down completely and is no longer releasing any hormones, the womb sheds its lining. This is your period. And so begins your next cycle.