Periods and puberty

The menstrual cycle starts during puberty. It is the cause of ‘periods’, the bleeding that most women experience about once a month. The proper word for having periods is menstruation.

Menstruation is a natural part of the menstrual cycle, normally happening about every 28 days, with bleeding normally lasting for 2-7 days.  Having regular menstrual cycles suggests that the hormones in the body are working as they should and is a sign of good health.

The experience of menstruation can be very different for different people, with some people having longer, shorter, or irregular cycles. But anyone having periods more than 35 days apart (or fewer than 8 periods in a year) should get checked out by a doctor. Also, anyone with very heavy, painful or otherwise troublesome periods should ask a doctor for help, as there are treatments that can help.

Pregnancy preparation

The menstrual cycle is all about getting the body ready for a possible pregnancy. This preparation involves a number of organs working together.

The most important cell in this story is the egg cell. And every egg starts in an ovary.

There are two ovaries, one on each side of the body. They are about 4 cm long and shaped a bit like eggs. The ovaries are held in place by stalk-like ligaments sticking out from each side of the uterus. The uterus (also known as the womb) is the organ where a baby might grow if the egg was fertilised to form an embryo.

Each ovary sits very close to an oviduct (also known as a fallopian tube). The oviducts are attached to the uterus too, but this isn’t just to keep them in place. The oviducts are tubes, which start very close to the ovary and lead all the way down into the uterus.

Egg cells are released into the oviducts when they are mature. This step is called ovulation. The oviduct directs the egg down into the uterus where it will need to attach (implant) to start a pregnancy. It would also need to be fertilised by sperm, which usually happens during the journey down the oviduct.

The uterus grows a thick lining in preparation for a possible pregnancy. If there is no pregnancy, this lining isn’t required any more and is lost as menstrual bleeding. 

Hormones

For all of this to work, the organs need to get their timings synchronised. The uterus needs to be ready when the egg arrives, and the ovaries mustn’t release eggs before they are ready, or release too many.

These timings are controlled by hormones. Hormones are chemical messengers that travel between organs. They are great for sending signals to lots of different cells all at once.

There are many different hormones involved in the menstrual cycle. The levels of these hormones rise and fall at different times. This can increase or decrease different cells’ activity.

Four key hormones help direct the menstrual cycle through the signals they carry:

Follicle Stimulating Hormone (FSH) – “Start growing eggs!” 

Oestrogen – “Eggs are growing!”

Luteinising Hormone (LH) – “Release the egg!” 

Progesterone – “The egg has been released!” 

Step 1: Grow the egg!

The first step is to grow and mature an egg ready for release. This step is controlled by follicle stimulating hormone (FSH).

FSH is produced and released by the pituitary gland  in the brain. It has this name because the first step of egg development involves growing a follicle (a fluid-filled sac) around the egg.

As these follicles grow, they release the hormone oestrogen.

Oestrogen’s effects include signalling for the pituitary to stop making FSH. This stops too many follicles growing. Normally only one follicle grows to maturity each month. But sometimes there are more - this can result in non-identical twins, or even triplets!

The same increase in oestrogen also signals to the uterus that it should start growing its lining.

Step 2: Release the egg! Ovulation

As the follicles grow larger, they release more and more oestrogen. When the oestrogen signal gets big enough, the pituitary gland releases luteinising hormone (LH). This hormone signals that it’s time for ovulation.

Ovulation is the moment that a mature egg cell is released from the ovary. It pops out of its follicle and is caught by the fringed opening of the oviduct. From there it travels down the oviduct and into the uterus. The rest of the follicle is left behind in the ovary.

The name of Luteinising Hormone (LH) comes from its effect on the egg’s now-empty follicle. When it detects the hormone, it changes its appearance and goes a bit yellow (“Luteus” is Latin for “yellow”). The ex-follicle now has an important new role as the corpus luteum (the “yellow body”). 

Step 3: The egg has been released!

The corpus luteum (the transformed follicle) now produces the hormone progesterone. This signals that ovulation has occurred and an egg is on its way to the uterus.

Having already detected the earlier oestrogen signals, the uterus has been growing its lining in anticipation of an egg being released. When it receives the progesterone signal, it speeds up to complete the job. The uterus is now ready to welcome an embryo and support a possible pregnancy.

Meanwhile in the pituitary gland, the progesterone signal stops the release of FSH and LH. This is important. The sudden surge in progesterone must mean that an egg has already been released this cycle. Stopping FSH and LH prevents any more eggs developing and being released.

Step 4: Pregnancy or period

What happens next depends on whether there is a pregnancy or not.

If the egg is fertilised and the embryo successfully attaches to the lining of the uterus, then there is a pregnancy. The embryo sends out its own hormonal signals, including ones which tell the Corpus Luteum to continue making progesterone. This is important as the progesterone signals to the uterus that there is a pregnancy for it to support.

If there is no pregnancy, there is no signal to the Corpus Luteum. It stops making progesterone. And without the progesterone signal, the uterus does not keep its lining. The lining tissue is shed and flows out of the body through the vagina. This is menstruation.

Step 1 (again): A new cycleAfter menstruation, the cycle starts all over again with the pituitary gland releasing Follicle Stimulating Hormone (FSH). The developing follicles again start to release oestrogen, which causes the uterus to start re-growing its lining.

Hormonal contraceptive pills

Hormonal contraceptive pills work by changing the level of hormones in the body. They boost levels of oestrogen or progesterone. High levels of these hormones normally signal that eggs are mature and/or have been released.

The pituitary gland responds to this by stopping production of FSH and LH – the hormones that drive egg growth and release. Without FSH and LH, no follicles grow, no eggs are released, and so there can be no pregnancy.

Hormonal contraceptives are more than 90% effective if taken correctly.

Menopause

Most women continue having menstrual cycles until later life. The cycle then stops as the levels of the hormone oestrogen decrease. This is called menopause, but the cycle doesn’t just pause - it stops for good. Some women take hormonal treatments to help with the symptoms of menopause.

Find out more: About the Menopause.