Menstrual Health Centre

The menstrual cycle, ovulation and fertility

Menstrual cycle Print

 Why we have periods

As we grow, our bodies change so that we can have children - if the time is right, and if we want to. We see some of these changes all the time. Like the way breasts grow larger as girls become women. And the way our hips widen in our teens.

Although we can’t see it, changes happen inside us, too. One of the changes involves the female sex hormone oestrogen. It’s called a ‘sex’ hormone because it plays a big role in making us able to sexually reproduce - or have children. As girls enter their teens they begin to make higher levels of oestrogen, as their bodies prepare them for becoming women.

Oestrogen plays a big part controlling the number of periods we have, and how long we have them for. Today, girls start to produce high levels of oestrogen much younger than they did 100 or 200 years ago, mostly because they have better diets and richer food. Why? Well, fat cells produce more oestrogen - it’s really that simple1. Today, the average British woman has about 450 periods over her life. In poorer countries, women can have as few as 150. In Stone Age times, women could have just 50 periods in their lifetime2.

Oestrogen starts to alter the way our bodies work. Take the ovaries, the two parts of the body in our bellies that hold all of a woman’s eggs. (The term comes from the Latin, meaning ‘egg keeper’3- doctor’s love fancy names.) When we’re born, we already have all the eggs - or ‘ova’ we will ever use in our lifetimes, stored in the ovaries. (It’s about 2 million, although it decreases to 400,000 when we become teenagers4.)

As we enter puberty, or the time a girl starts to develop into an adult woman, the ovaries begin to change. It’s not too simplistic to say: it’s just like they’re ‘waking up’ and becoming active. (You can’t blame them: they’ve waited ages.)

Our ovaries start to release eggs into the womb, or uterus - one ovary releasing an egg one month, and the other ovary releasing an egg the next. It’s called ovulation. For most of us, this begins to happen between the ages of 11 to 14, although everyone’s a little different. When we’re young, our eggs can be released at irregular times. But by our late teens, most of us find that an egg is released every 28 days.

Every month, before an egg is released, the womb - or uterus - begins to prepare for it. It builds up a thicker lining around its walls, a little like a cushion. This is the endometrium. If you feel twinges or pains in the days before your bleeding starts, you may be feeling this lining build up.

When the egg is finally released, it sticks to this new lining. If we’re old enough to have sex, the egg may be fertilised, where the sperm from a man enters the egg and triggers its development into an embryo - a ball of cells that may become a baby. Some embryos don’t grow for very long, but others can to develop into a foetus - the earliest stage of pregnancy.

Most eggs aren’t fertilised. When the egg is unfertilised, the uterus begins to lose its lining after a few days. We experience this as the blood flow that comes once a month. Women recognise this bleeding as menstruation, or our period.

But, how does it work? The body releases hormones called prostaglandins. Prostaglandins are one of the ways our bodies send signals or instructions to our organs, the parts of the body - like heart, womb and ovaries - that have specific jobs. In this case, they signal the muscles in the womb to contract. And these contractions push the lining out of the body, through the vagina.

Some of us have bodies that don’t make many prostaglandins, so the contractions won’t be very strong. And some women don’t really have period bleeding, because their wombs absorb the lining. It’s called endometrial sparing5. (In some ways, it’s the ultimate in recycling.)

If we produce higher levels of prostaglandins, it’s a bit like yelling at someone through a megaphone - telling them to really get going. It makes our wombs contract harder and faster, causing aches and cramps. Because the muscles in the womb are strong enough to push out a baby, some women can experience severe pain, called dysmenorrhea6 - although most of us don’t have pain that’s this bad. To learn more about the more painful side of periods - and how to cope with them - look at our pages on pre menstrual syndrome or PMS, menstrual migraines, and preventing pain.

As women, our prostaglandins take on a different role as we enter puberty, or the time when we start to develop ‘sexual characteristics’ like breasts and pubic hair. They’re called ‘sexual characteristics’ because they show that we are becoming capable of sexual reproduction, or in ordinary language, having babies. In fact, oestrogen - the female ‘sex hormone’ - plays a big part in helping our bodies make prostaglandins.

We can have periods from puberty until the menopause. All of our bodies are different, so some of us become fertile, or able to have children, earlier than others. It doesn’t mean that we should start to have children - just that our bodies are prepared for it.

If you’d like to learn more about your body’s monthly cycle, take a look at our animation, Your Menstrual Cycle.

You're not alone

We enter the menopause at different times, too - the stage in our older lives when we lose the ability to become pregnant. For most women, the menopause happens in the late 40s to early 50s, but there can be a lot of variation.

You can find out about the menopause here.

  1. 1 Livoti, Dr. Carol, and Elizabeth Topp. 2004. Vaginas: An Owner’s Manual. New York, NY: Thunder’s Mouth Press
  2. 2 Livoti, Dr. Carol, and Elizabeth Topp. 2004. Vaginas: An Owner’s Manual. New York, NY: Thunder’s Mouth Press.
  3. 3 Grahn, Judy. 1993. Blood, Bread, and Roses: How Menstruation Created the World. Boston, MA: Beacon Press.
  4. 4 Daniels, Patricia, et al. 2007. Body: The Complete Human. Washington, D.C.: National Geographic Society.
  5. 5 Livoti, Dr. Carol, and Elizabeth Topp. 2004. Vaginas: An Owner’s Manual. New York, NY: Thunder’s Mouth Press.
  6. 6 Livoti, Dr. Carol, and Elizabeth Topp. 2004. Vaginas: An Owner’s Manual. New York, NY: Thunder’s Mouth Press.

Premenstrual Syndrome, or PMS

What is the Menopause image

Don’t suffer alone. If you find yourself - or your body - griping and grumping in the week or so before your period, you may have Premenstrual Syndrome, or PMS. You’re not alone, because PMS is surprisingly widespread...

Menstrual Health Centre

Go to the Menstrual health centre

symptoms to spot
– and save a life

Read more

Your period is as individual
as your body...

Our eggs are the largest cells in our bodies, and are the only ones that we can see without a microscope.