Chapter 6 of 8

All 8 chapters
  1. 1The human life cycle, readunderstood
  2. 2Changes in puberty, readunderstood
  3. 3Feelings and growing up, readunderstood
  4. 4Reproductive systems, readunderstood
  5. 5The menstrual cycle, readunderstood
  6. 6Fertilisation and pregnancy, readunderstood
  7. 7Birth and caring for a baby, readunderstood
  8. 8Staying healthy and safe, readunderstood

S3.4.6 Chapter 6

Fertilisation and pregnancy

How does a baby start, and how does it grow inside the mother?

Your guess

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

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how a sperm and an egg join to make a new cell
  • Describe how a baby grows in the uterus over about nine months

The egg and the sperm are special cells, called sex cells. Each has a nucleus with half the instructions for a new person. Joined together, they make a full set.

An egg cell and a sperm cell drawn at their true sizes, side by side. The egg is a large circle, and a bar under it is as long as the egg is wide, marked 0.1 mm. The sperm is a tiny dark head with a thin wavy tail, about half as long as the egg is wide. Labels: egg cell, sperm cell, 0.1 mm.egg cellsperm cell0.1 mm
True sizes. The egg is about 0.1 mm across, about the smallest dot an eye can see. It is about 20 times as wide as a sperm's head is long.
An egg cell and a sperm cell with their parts named. The egg has a jelly coat round the outside, then a cell membrane, then the food store, with the nucleus inside. The sperm, drawn much bigger than it really is, has a head holding its nucleus and a long tail. Labels: jelly coat, cell membrane, nucleus, food store, head, tail.jellycoatcellmembranenucleusfoodstoreheadtail
The egg has a big food store for the first days. The sperm has a tail to swim with. Here the sperm is drawn much bigger than it is.

Sexual intercourse, or sex, is when an erect penis goes into the vagina. Semen comes out of the penis into the vagina. Both people must freely agree. A child cannot agree to sex, and sex with a child is always wrong. It is never the child's fault.

Draw it first

Draw how the sperm meets the egg

  1. The female reproductive system from the front, drawn three times, one picture for each step. Step one: an egg is just leaving the ovary on the right, and a dashed arrow runs from that ovary into the open end of the oviduct and along it. Step two: the egg is in the right oviduct, and a dashed arrow runs from the vagina up through the cervix and the uterus into that oviduct. Step three: the egg is in the oviduct, where one sperm joins it, and the lining of the uterus is thick. Labels: ovary, egg, oviduct, vagina, cervix, uterus, lining.ovaryeggoviduct

    1 An ovary lets go of an egg. It moves into the oviduct.

  2. The female reproductive system from the front, drawn three times, one picture for each step. Step one: an egg is just leaving the ovary on the right, and a dashed arrow runs from that ovary into the open end of the oviduct and along it. Step two: the egg is in the right oviduct, and a dashed arrow runs from the vagina up through the cervix and the uterus into that oviduct. Step three: the egg is in the oviduct, where one sperm joins it, and the lining of the uterus is thick. Labels: ovary, egg, oviduct, vagina, cervix, uterus, lining.vaginacervixuterusegg

    2 Sperm swim from the vagina, through the cervix and the uterus, into the oviducts.

  3. The female reproductive system from the front, drawn three times, one picture for each step. Step one: an egg is just leaving the ovary on the right, and a dashed arrow runs from that ovary into the open end of the oviduct and along it. Step two: the egg is in the right oviduct, and a dashed arrow runs from the vagina up through the cervix and the uterus into that oviduct. Step three: the egg is in the oviduct, where one sperm joins it, and the lining of the uterus is thick. Labels: ovary, egg, oviduct, vagina, cervix, uterus, lining.eggoviductlining

    3 In the oviduct, one sperm may join the egg. The lining of the uterus is thick and ready.

Millions of sperm set off, but only a few hundred reach the egg. The head of one sperm gets into the egg. Then the egg changes, so that no other sperm can get in. The nucleus of the sperm joins the nucleus of the egg. This is fertilisation.

Four boxes, one under another, joined by arrows going down: fertilised egg, two cells, ball of cells, settles in lining. Labels: fertilised egg, two cells, ball of cells, settles in lining.fertilised eggtwo cellsball of cellssettles in lining
The fertilised egg divides again and again as it moves down the oviduct. About a week later, the ball of cells settles in the soft lining.

A girl can become pregnant the first time she has sex. She can even become pregnant before her first period. That is because an egg is let go about two weeks before the period comes.

Pregnancy can be prevented. The ways to do this are called contraception. Sex can also pass on infections, such as HIV. The book Health, disease and immunity explains both, and a health worker can advise you.

Settling in the lining starts a pregnancy. While a woman is pregnant, she has no periods, because the lining is needed. A pregnancy lasts about 40 weeks, which is about nine months. It is counted from the first day of the mother's last period. The baby itself grows for about 38 weeks. In the first weeks it is called an embryo. From about 10 weeks of pregnancy it is called a fetus.

A standing person seen from the front. Inside the abdomen and pelvis is a large uterus. Inside it, the baby lies head down in the amniotic sac, full of amniotic fluid. The placenta is on the wall of the uterus, and the umbilical cord runs from the placenta to the baby's belly. The cervix closes the bottom of the uterus, and the vagina runs from it to the outside. Labels: uterus, fetus, cervix, vagina, placenta, umbilical cord, amniotic sac, amniotic fluid.uterusplacentaumbilicalcordamnioticsacamnioticfluidfetuscervixvagina
A baby late in pregnancy. It floats in fluid, inside a bag called the amniotic sac. The fluid cushions it from bumps.

The placenta grows on the wall of the uterus. The umbilical cord joins the baby to it. In the placenta, food and oxygen pass from the mother's blood into the baby's blood. Waste, such as carbon dioxide, passes the other way. The two bloods stay apart and do not mix.

See it three ways

As a picture

Four boxes, one under another, joined by arrows going down: mother's blood, placenta, umbilical cord, baby. Labels: mother's blood, placenta, umbilical cord, baby.mother's bloodplacentaumbilical cordbaby

In words

Food and oxygen go from the mother's blood, through the placenta and along the cord, to the baby. Waste goes back the other way.

In symbols

40 weeks × 7 = 280 days

A centimetre ruler from 0 to 50 cm, and under it four babies lying stretched out, head at 0, each with an exact bar as long as its length. At 8 weeks, about 2 cm. At 20 weeks, about 25 cm. At 30 weeks, about 40 cm. At 40 weeks, about 50 cm. Labels: 8 weeks, 20 weeks, 30 weeks, 40 weeks.01020304050cm8 weeks2 cm20 weeks25 cm30 weeks40 cm40 weeks50 cm
How long the baby is, from head to heel, at four stages. The weeks are counted from the mother's last period. Every baby is a little different.

Try it

Make a size line of a growing baby

You need: paper, a pencil, a ruler.

Be safe: Take care if you use scissors.

  1. Tear or cut strips of paper 2 cm, 25 cm, 40 cm and 50 cm long. Join sheets if you need to.
  2. Write on each strip how many weeks it shows: 8, 20, 30 and 40 weeks.
  3. Lay the strips side by side, shortest first. Compare the gaps between them.

What you will see: The baby gets longer fastest in the middle weeks. Near the end it grows less in length, but it puts on most of its weight.

If you cannot try it, this is what happens.
A table of the paper strips. 8 weeks: 2 cm. 20 weeks: 25 cm. 30 weeks: 40 cm. 40 weeks: 50 cm. Labels: weeks, length, 8, 20, 30, 40, 2 cm, 25 cm, 40 cm, 50 cm.weekslength82 cm2025 cm3040 cm4050 cm

A wrong idea many people have

A baby in the uterus eats and breathes through its mouth.

Why it feels right: After birth a baby feeds through its mouth and breathes air.

The test that breaks it: Look at the drawing. The baby floats in fluid, with no air and no food round it.

What is true: Before birth, food and oxygen come from the mother's blood, through the placenta and the cord.

Key words

sex cells
the egg and the sperm, each with half the instructions for a new person.
sexual intercourse
when an erect penis goes into the vagina. Both people must freely agree.
fertilisation
when the nucleus of a sperm joins the nucleus of an egg.
pregnancy
the time a baby grows in the uterus, about 40 weeks counted from the last period.
placenta
the organ on the wall of the uterus where food and oxygen pass to the baby.
umbilical cord
the cord that joins the baby to the placenta.

Back to your guess

The answer is A: A baby starts when one sperm joins with one egg.

A baby starts when one sperm joins one egg. Each brings half the instructions. The new cell then divides again and again, and the parts of the baby form over many weeks.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What is fertilisation?

    Tap your answer.

    1. Not this one. Only one sperm gets in. Then the egg changes so no other can. Go to S3.4.6. Try another.

    2. Yes, that is right. Each brings half the instructions, so together they make a full set for a new person.

    3. Not this one. That comes about a week later, and it starts a pregnancy. Go to S3.4.6. Try another.

    See the answer

    The nucleus of one sperm joins the nucleus of one egg.

    Each brings half the instructions, so together they make a full set for a new person.

    • If you chose A: Only one sperm gets in. Then the egg changes so no other can. Go to S3.4.6.
    • If you chose C: That comes about a week later, and it starts a pregnancy. Go to S3.4.6.

  2. Where do a sperm and an egg usually meet?

    Tap your answer.

    1. Not this one. Sperm are placed in the vagina, but they swim on, through the uterus, into the oviducts. Go to S3.4.6. Try another.

    2. Not this one. The egg leaves the ovary first. They meet in the oviduct. Go to S3.4.6. Try another.

    3. Yes, that is right. The egg moves along the oviduct, and the sperm swim up into it. That is where they meet.

    See the answer

    In an oviduct

    The egg moves along the oviduct, and the sperm swim up into it. That is where they meet.

    • If you chose A: Sperm are placed in the vagina, but they swim on, through the uterus, into the oviducts. Go to S3.4.6.
    • If you chose B: The egg leaves the ovary first. They meet in the oviduct. Go to S3.4.6.

  3. How does a baby in the uterus get oxygen?

    Tap your answer.

    1. Not this one. There is no air in the uterus. The baby floats in fluid. Go to S3.4.6. Try another.

    2. Yes, that is right. In the placenta, oxygen passes from the mother's blood into the baby's blood, which flows along the cord.

    3. Not this one. Every living cell needs oxygen. The baby gets it through the placenta. Go to S3.4.6. Try another.

    See the answer

    From the mother's blood, through the placenta and the cord

    In the placenta, oxygen passes from the mother's blood into the baby's blood, which flows along the cord.

    • If you chose A: There is no air in the uterus. The baby floats in fluid. Go to S3.4.6.
    • If you chose C: Every living cell needs oxygen. The baby gets it through the placenta. Go to S3.4.6.

  4. How long does a pregnancy usually last?

    Tap your answer.

    1. Not this one. That is far too short. A baby needs about 40 weeks to grow. Go to S3.4.6. Try another.

    2. Not this one. After 4 weeks the embryo is still tiny. A baby needs about 40 weeks. Go to S3.4.6. Try another.

    3. Yes, that is right. A pregnancy lasts about 40 weeks, about nine months, counted from the first day of the last period.

    See the answer

    About 40 weeks

    A pregnancy lasts about 40 weeks, about nine months, counted from the first day of the last period.

    • If you chose A: That is far too short. A baby needs about 40 weeks to grow. Go to S3.4.6.
    • If you chose B: After 4 weeks the embryo is still tiny. A baby needs about 40 weeks. Go to S3.4.6.

  5. What does the amniotic fluid do?

    Tap your answer.

    1. Yes, that is right. The baby floats in the fluid, which soaks up knocks and keeps it safe.

    2. Not this one. Food comes through the placenta and the cord. The fluid protects the baby. Go to S3.4.6. Try another.

    3. Not this one. Oxygen comes from the mother's blood, through the placenta. Go to S3.4.6. Try another.

    See the answer

    It cushions the baby from bumps.

    The baby floats in the fluid, which soaks up knocks and keeps it safe.

    • If you chose B: Food comes through the placenta and the cord. The fluid protects the baby. Go to S3.4.6.
    • If you chose C: Oxygen comes from the mother's blood, through the placenta. Go to S3.4.6.

  6. Why does a pregnant woman have no periods?

    Tap your answer.

    1. Yes, that is right. A period is the lining breaking down. In a pregnancy the lining is kept, so there are no periods.

    2. Not this one. Her body pauses the cycle while the baby grows. It does not stop for ever. Go to S3.4.5. Try another.

    3. Not this one. Period blood comes from the lining of the uterus. The lining is kept for the baby. Go to S3.4.5. Try another.

    See the answer

    The lining is needed, because the baby is growing in it.

    A period is the lining breaking down. In a pregnancy the lining is kept, so there are no periods.

    • If you chose B: Her body pauses the cycle while the baby grows. It does not stop for ever. Go to S3.4.5.
    • If you chose C: Period blood comes from the lining of the uterus. The lining is kept for the baby. Go to S3.4.5.

  7. A pregnancy lasts about 40 weeks. How many days is that?

    See the answer

    About 280 days

    A week has 7 days. 40 × 7 = 280.

  8. At 30 weeks a baby is about 40 cm long. At 40 weeks it is about 50 cm long. How much longer does it grow?

    See the answer

    About 10 cm

    50 − 40 = 10.

  9. From an earlier chapter

    What happens to the lining of the uterus if an egg is not fertilised?

    Tap your answer.

    1. Not this one. If no baby starts, the lining is not needed, so it breaks down. Go to S3.4.5. Try another.

    2. Yes, that is right. The lining builds up for a fertilised egg. With none, it breaks down and leaves through the vagina.

    3. Not this one. Eggs come from the ovaries, not from the lining. Go to S3.4.5. Try another.

    See the answer

    It breaks down and leaves the body as a period.

    The lining builds up for a fertilised egg. With none, it breaks down and leaves through the vagina.

    • If you chose A: If no baby starts, the lining is not needed, so it breaks down. Go to S3.4.5.
    • If you chose C: Eggs come from the ovaries, not from the lining. Go to S3.4.5.

Keep it

Copy this into your exercise book.

One sperm joins one egg in the oviduct: fertilisation. The new cell divides and settles in the lining. The baby grows for about 40 weeks. The placenta and the cord bring it food and oxygen.

Draw: Draw a baby in the uterus with the placenta, the cord and the fluid.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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