Kids' Health
Visit website  
Home › Topics › Nearly Teens > 

Sexual reproduction - how babies are made - for kids

sperm; sexual; uterus; pregnant; placenta; umbilical; cord; reproduction; sex; baby; babies; foetus; fetus; embryo; intercourse; fertilisation; sperm; egg; ovum; cell; womb; chromosome; gene; DNA; twins ;


When girls and boys reach puberty, their bodies start to change and become more mature. From this time, if a male and a female have sexual intercourse (often called 'making love', or 'sleeping with someone'), it is possible that the girl could get pregnant, ie. a baby could start to grow.

How a baby is started

Sperm are the male 'seeds' that contribute to starting a new life - living sperm look a bit like tadpoles (under a microscope). They form in the testes.

sexualityWhen sperm are ejaculated (say ee-jak-u-lay-ted) from the penis during sexual intercourse, they swim up the vagina (vaj-eye-na), through the cervix (sir-vix), into the uterus (you-ter-us) and then into the fallopian (fal-o-pe-an) tubes of the female. These sperm are looking for an ovum (or egg) to fertilise.

When a female is born she carries thousands of ova or eggs ready to use when she becomes adult. These are the female 'seeds' that, along with sperm, create a new life.

sexualityOnce a month, the female releases an ovum (one egg) or sometimes two (ova) .

If an ovum has been released, and the couple have sex, a sperm can unite with it, fertilise it and make the first cell of a new baby.

Once one sperm has fertilised the ovum, no other sperm can get in.

For the sperm it's like a race and there is only one winner.

Ovulation and menstruation [periods] can begin as early as 9 years for some females, but usually start at a few years later for most girls. Have a look at this topic on Periods. A female is able to get pregnant once she starts ovulating. When a girl starts having periods it is the sign that she has started ovulating. Young girls are not physically and emotionally capable of dealing with pregnancy and being a mother until they become young  adults.

The law: In South Australia it is against the law to have sexual intercourse when you are under 17 years old. These laws are to protect young people from getting hurt by situations that they aren't old enough or experienced enough to manage.

What happens next

This fertilised ovum immediately divides into two cells, these cells then divide again and again over the next couple of days as the cluster of cells makes its way to the uterus (womb). Here it is planted in the lining of the uterus and continues dividing its cells to make billions of new cells. The female is now pregnant.

The amazing thing is that each one of these cells contains the same set of chromosomes or 'plans' that were created at fertilisation!

Over 9 months, these cells will grow into a new person - a baby.

Doctors have different names for this developing baby.

  • 1 day - 'zygote'
  • first 2 months - 'embryo'
  • 3rd month to birth - 'fetus'

When your dad's sperm and mum's egg (ovum) got together, they each brought a set of 'plans' for what the new baby would be like.

When the ovum was fertilised and became your first cells, these 'plans' or genes helped to decide lots of things about you, eg. boy or girl, colour of skin, eyes, hair, etc.

Genes are made of DNA ('de-ox-y-ri-bo-nu-cle-ic acid', if you want the full name).

If you could see your genes they would look like beads on a necklace of DNA. These strands are called 'chromosomes'.

Each cell in a human body has 46 chromosomes.

That first single cell has 23 chromosomes from mum and 23 from dad, which is why you might look like mum or dad (or grandparents) and have similar traits, eg. you and dad may have pointy ears, or you and mum can both wiggle your noses!

(Isn't it annoying when people who haven't seen you for a while say things like, "He's got his dad's chin", or "she's got grandma's eyes!" Or you might find it very interesting.)

Twins and more

  • Remember, any one sperm can only fertilise one ovum, so if 2 ova (eggs) leave the ovaries at the same time and are both fertilised then 'non-identical' twins are born. They may look alike or they may not, just like any brothers or sisters.
  • If an ovum splits after it has been fertilised, then you get identical twins because they have the same set of genes.
  • It is called a multiple birth if two or more babies are born at the same birth. Do you know what we call a set of three babies who are born at the same time?

What sex you are

What sex a baby will be is decided when the egg and sperm unite.

Each egg and each sperm have one sex chromosome.

There are two kinds of sex chromosomes - X and Y. Can you see why they are called x and y chromosomes? (Hint - look at the picture of a male chromosome.)


Eggs carry only an X and sperm carry either an X or a Y

sexualityX+X means the cell will develop into a baby girl.
X+Y means that it will develop into a boy.

Once you are born, you will grow up into a unique  human being - there's no-one else like you in the world. Even identical twins are not exact copies of each other - they each have their own personalities.

You may look a bit like someone in your family, but there is only one of you!

You are a completely unique and wonderful person.

There is a lot more about this in the topic Genes - not the kind you wear.

Inside the uterus (womb)

The place where the embryo plants itself is inside the uterus. The baby starts to grow, and other tissue grows into a placenta (say pla-sent-a).

female organsDuring pregnancy (the time when the baby is growing in mum's uterus), the placenta provides oxygen from the air that mum breathes, and nutrients (say new-tree-ents) from the food she eats.

This is why it is important that mum gets good food and takes care not to smoke, drink alcohol or take drugs, because the developing baby gets those too and he or she cannot say, "No".

Nutrients from what mum eats or drinks, and oxygen from the air she breathes, go through the umbilical cord to the fetus. Any waste from the growing baby goes back through the cord into the mother's bloodstream and passes out of her body.

The umbilical cord is a soft 'bendy' tube from the placenta to the navel (or tummy button) of the fetus.

There is a sac (a bag) of thin skin filled with fluid protecting the skin of the developing baby. The baby can move around safely inside the mother for 9 months until he or she is ready to be born into our world.

Dr Kim says

Dr Kim
"When a girl starts menstruating (gets her period), that means that she is able to conceive (get pregnant). Having a baby means that the mum and dad have to be ready to look after their child until that child becomes an adult. It is a big decision that will affect the rest of their lives and that of their baby."

 In Australia it is against the law to have sexual intercourse when you are under 17 years old or with someone who is under 17. These laws are to protect young people from getting hurt by situations that they are not old enough or experienced enough to manage.


"My sister got pregnant when she was 15. My mum and dad were very upset but they helped her. She had the baby and he lives with us now. Some people said mean things to my sister and to our family. It is still hard for our family, and my sister had to leave school. She wants to go back to school when her baby is older. My sister told mum that there was another girl in the hospital who had to let her baby be adopted by another family because she didn't have a mum and dad who would help her."

back to top

We've provided this information to help you to understand important things about staying healthy and happy. However, if you feel sick or unhappy, it is important to tell your mum or dad, a teacher or another grown-up.


Home › Topics › Nearly Teens >