S4.5.7 Chapter 7

Hormones and the menstrual cycle

How does the body know when to release an egg, and when to start a period?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Nerves carry signals from the brain to the uterus.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Hormones in the blood, from the pituitary gland and the ovaries, switch each other on and off.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The uterus keeps its own count of the days.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how hormones control the menstrual cycle
Explain how hormones are used in contraception and fertility treatment

From puberty until periods stop later in life, the female body goes through a cycle about once a month. It is the menstrual cycle. Its job is to release an egg and get the uterus ready, in case a pregnancy starts.

The two ovaries hold the eggs. The lining of the uterus, drawn dotted, builds up and breaks down in each cycle.

A cycle is about 28 days long on average. Anything from about 21 to 35 days is common. Cycles differ between people, and from one month to the next. In the first years after periods start, they are often irregular.

Day 1 of a cycle is the first day of bleeding. The lining of the uterus breaks down and leaves through the vagina as blood. This is menstruation, also called a period. Most periods last about 2 to 7 days.

After the period, the lining builds up again. Around day 14 of a 28 day cycle, one ovary releases an egg. This is ovulation. The egg moves along the oviduct toward the uterus.

If the egg is not fertilised, it dies within about a day. The lining is kept for about two more weeks, then it breaks down. That is the next period, and a new cycle starts.

If the egg is fertilised and settles into the lining, a pregnancy starts. The lining is kept, so there are no periods during the pregnancy.

A 28 day cycle. The period here is days 1 to 5, and the egg is released about day 14.

Ovulation comes about 14 days before the next period starts. So in a longer cycle, ovulation comes later, and in a shorter cycle it comes sooner.

Worked example

Amina's cycles are about 26 days long. About which day of her cycle is ovulation?

Ovulation comes about 14 days before the next period starts.
26 − 14 = 12.

Answer: About day 12

Amina's 26 day cycle. The egg is released about day 12, still about 14 days before the next period.

These days are averages. The real day of ovulation can move by several days, even in the same person. So counting days alone cannot say for sure when an egg is released.

What controls all this? Four hormones. Two are made by the pituitary gland, a small gland under the brain. Two are made by the ovaries. All four travel in the blood.

The glands that control the cycle. Hormones from the pituitary gland reach the ovaries in the blood. Hormones from the ovaries reach the pituitary gland the same way.

FSH (follicle stimulating hormone) is made by the pituitary gland. It makes an egg mature inside the ovary. It also makes the ovary produce oestrogen.

Oestrogen is made by the ovaries. It makes the lining of the uterus build up again after a period. As it rises, it holds back FSH, so no more eggs start to mature.

When oestrogen is high, it makes the pituitary gland release a sudden burst of LH (luteinising hormone). This burst is the LH surge. It causes ovulation.

After ovulation, the part of the ovary that held the egg makes progesterone, and some oestrogen. Progesterone keeps the lining thick and ready. It also holds back FSH and LH, so no new egg matures.

If no pregnancy starts, progesterone falls near the end of the cycle. Without it, the lining breaks down and the period starts. FSH is no longer held back, so it rises and a new cycle begins.

Each hormone is drawn on its own scale, so the graph has no unit. Compare the shapes and the timing, not the heights. Real levels differ between people.

Read the graph from left to right. FSH is high early on. Oestrogen peaks just before the LH surge. The LH surge peaks just before day 14, and ovulation follows. Progesterone is high in the second half.

The lining on day 22, thick and ready, held there by progesterone.

The lining is shed during the period, built up by oestrogen, then kept thick by progesterone.

Each hormone switches the next one on or holds another back. That is why the cycle repeats.

Try it

Draw two made-up cycles on paper

You need: paper, a pencil, a ruler.

Draw a row of 28 boxes and number them 1 to 28. This is a made-up 28 day cycle.
Shade boxes 1 to 5 for the bleeding. Write "lining builds" over boxes 6 to 13. Draw a small egg in box 14.
From the graph in this chapter, write oestrogen over box 12, LH over box 13 and progesterone over box 21. Each is highest there.
Draw a second row of 32 boxes. The egg goes in box 18, because 32 − 14 = 18. Count the boxes after the egg in each row.

What you will see: Both rows have 14 boxes after the egg. In the longer cycle the first half is longer, but the second half stays about 14 days.

If you cannot try it, this is what happens.

Contraception means ways to prevent pregnancy. Some ways use hormones. The hormones in them are made to work like the body's own oestrogen and progesterone.

The combined pill contains oestrogen and progesterone. Taken as a health worker explains, these hormones hold back FSH and LH. So no egg matures, and none is released. The pill also thickens the mucus at the cervix.

How the combined pill stops ovulation. It uses the same holding back the body's own hormones do in every cycle.

Progesterone only methods include a pill, an injection, a small implant under the skin of the arm, and a hormonal coil. A health worker places the coil in the uterus.

All of them thicken the mucus at the cervix, so sperm cannot easily get through. Some also stop ovulation, and the hormonal coil keeps the lining thin.

Every progesterone only method does this. The injection, the implant and some pills also stop ovulation.

Used correctly every time, hormone methods are very effective. A missed or late pill makes the pill less reliable. An implant, an injection or a coil does not need remembering each day, so it fails less often.

Hormone methods can have side effects, such as changes in bleeding, headaches or changes in mood. These differ between people. Rarely, combined methods cause blood clots, so a health worker checks health first. A health worker can help choose a method that suits.

No hormone method protects against sexually transmitted infections. Only condoms also give that protection.

Some people find it hard to become pregnant. One cause is that eggs do not mature, or are not released. Hormones can help with this.

In fertility treatment, a health worker can give FSH and LH. FSH helps eggs mature, and LH helps them be released. Several eggs may mature at once, so twins are more likely.

IVF (in vitro fertilisation) goes further. First FSH and LH are given, so several eggs mature. The eggs are collected from the ovaries with a fine needle.

In a dish in a laboratory, the eggs are mixed with sperm. Some are fertilised and grow for a few days into small balls of cells, called embryos. One embryo, or sometimes two, is placed in the uterus.

The steps of IVF. Fertilisation happens in the dish, outside the body.

Fertility treatment does not always work. Many tries do not lead to a pregnancy. It can be hard on the body and on the feelings, and the hormones can have side effects.

Periods are a normal, healthy part of life. They are not dirt, and they are not an illness.

Pads, cloths, tampons and cups, and how to use each safely, are in the Growing up chapter "The menstrual cycle". Some pain low in the belly is common.

Nobody may touch your body without your yes. That is true for everyone, every time. If anyone does, it is not your fault. Tell an adult you trust or a health worker.

A wrong idea many people have

A person cannot get pregnant during a period.

Why it feels right: Ovulation usually comes in the middle of the cycle, far from the period.

The test that breaks it: Sperm can live in the body for up to about 5 days. In a short cycle, ovulation can come soon after a period ends, while sperm are still alive.

What is true: Pregnancy is less likely during a period, but it can happen. Counting days alone is a less reliable way to avoid pregnancy.

Key words

menstruation: the lining of the uterus breaking down and leaving the body as blood; a period.
ovulation: an ovary releasing an egg, about 14 days before the next period.
FSH: a hormone from the pituitary gland that makes an egg mature.
oestrogen: a hormone from the ovaries that builds up the lining of the uterus.
LH: a hormone from the pituitary gland whose surge causes ovulation.
progesterone: a hormone from the ovaries that keeps the lining thick.
contraception: ways to prevent pregnancy.
IVF: fertility treatment in which eggs are fertilised with sperm in a dish.

Where to get help: a nurse, a doctor, a health worker at a clinic or a pharmacist. Each of them can answer questions about periods, contraception and fertility. An adult you trust can help you reach one. Health workers hear these questions every day.

Back to your guess

The answer is B: Hormones in the blood, from the pituitary gland and the ovaries, switch each other on and off.

Four hormones control the cycle. Two come from the pituitary gland and two from the ovaries. They travel in the blood and switch each other on and off, so the cycle repeats.

Keep it

Copy this into your exercise book.

FSH from the pituitary matures an egg. Oestrogen builds the lining and sets off LH. LH releases the egg. Progesterone keeps the lining thick; when it falls, a period starts. The pill holds back FSH and LH.

Draw: Draw the 28 day hormone graph with FSH, oestrogen, LH and progesterone, and mark the LH surge just before day 14.