A new human life begins
Have you ever rolled up into a ball? Perhaps it was in a PE lesson where the teacher said, "Make yourself as small as you can". In fact, you have been much smaller than that. Once you were just about the size of this full stop.

At that time, you looked like a tiny blob of jelly with a pale ball shape in the middle. Structures like this are called cells. The ball shape inside the cell is called the nucleus. It contains the instructions which control how the cell behaves. Life processes, which take place in the jelly part of the cell, are controlled by the instructions in the nucleus. Around the jelly is a skin called the cell membrane.

The nucleus of the first human cell or fertilised egg contains instructions for making every part of the body but it does not use them all at once. At first the nucleus makes the cell divide in two. When this happens, the nucleus divides too. Before the nucleus divides, threads called chromosomes appear. They are made of DNA and contain genes - the instructions of the cell. Each chromosome makes a copy of itself. When all the copies are made, the nucleus splits into two and each new nucleus has a set of chromosomes like the first one.

It is not long afterwards that each of the new cells divides. Their nuclei divide like the original one and four cells, each with a nucleus, are produced. Inside each nucleus is a set of chromosomes just like those in the original cell. The cells continue to divide and make a ball. Shortly afterwards, as the cells continue to divide, the human body begins to form. In some of the cells the nuclei send out instructions to make a head. In others instructions are sent out to make arms and legs. As the cells divide, some nuclei send out messages to make eyes, ears, fingers and toes. Inside the body, cells form organs such as the heart, lungs and stomach.

Eight weeks after the first cell started to divide all the organs have begun to develop and the body is two and a half centimetres long. In another two weeks, the body has grown to six centimetres long and even the finger nails and fingerprints have formed. The body continues to grow and develop over the following months.

People usually call the newly developing human a baby, but there are more precise terms that you might hear used by doctors and nurses. The developing baby up to eight weeks is called an embryo. After that time until the time of the birth the correct term for baby is foetus. By the time the foetus is twenty five weeks old the eyes open a little and the body is twenty five centimetres long. When the foetus is thirty eight weeks old it is forty eight centimetres long and ready to be born.

How is a fertilised egg cell made?
It is made when a sperm from the father joins with an egg from the mother. Both the sperm and the egg have a nucleus but each one has only half a set of chromosomes. The sperm and egg join together to make a fertilised egg, which then has a full set of chromosomes so a new human life can begin.

Why do the sperm and egg only have half a set of chromosomes each?
This reduction in the number of chromosomes occurs when they are made. If the sperm and egg had a full set of chromosomes, the fertilised cell would have two sets of chromosomes. This would produce a person with two sets of chromosomes. If people with two sets of chromosomes then produced fertilised eggs, the fertilised eggs would have four sets of chromosomes. If this doubling up continued, there would soon come a time when the nucleus was so large it would fill the cell and the cell would die.

Why can't people be produced from just a body cell without bothering with eggs and sperm?
If this were to happen you would just get an exact copy of the person. Very few living things produce exact copies of themselves because it is a threat to the survival of their species. Animals produce eggs and sperm because it gives them a chance to mix up the instructions a little and produce a variety of individuals. This increases the chance of survival. When eggs and sperm are being made, the chromosomes break up and rearrange the genes a little.

How does mixing up the genes increase the chance of survival?
Imagine an animal, which could only live at a certain temperature, such as 35°C. Generation after generation had exactly the same feature. As time went by, the temperature of the environment went up by 1°C. None of the animals could survive at this temperature so they all died. If the animals had mixed up their genes and bred using eggs and sperm some of the new individuals may have been able to survive at 35 and 36°C as well as just at 35°C. When the temperature of the environment went to 36°C there would still be some animals that could survive and thus prevent the animals becoming extinct.

How are twins formed?
Identical twins are produced from just one fertilised egg. When a fertilised egg divides, it produces two cells which stick together. Occasionally, the cell divides with such power that the two cells move apart. When this happens, each cell continues to divide on its own and each one forms a baby. Some twins are not identical. They are called fraternal twins and are produced by two separate eggs. Each one is fertilised by a sperm and the two eggs divide and grow inside the mother at the same time. They are not identical because the genes in one fertilised egg are slightly different from the genes in the other.

Where does the baby get its food from as it grows inside its mother?
When the fertilised egg first starts to divide, it uses energy and material stored in the cell. As the ball of cells forms, it sticks to the wall of an organ in the mother. This organ is called the womb. As the baby grows, it also forms a disc called a placenta. This plugs into the wall of the womb and food from the mother's blood passes into it. The baby is connected to the placenta by a cord called the umbilical cord. There are tubes called blood vessels in the placenta and umbilical cord. They are connected to the blood vessels and heart in the baby's body. The baby's heart pumps the blood through all these blood vessels. When the blood passes through the placenta it picks up the food that has passed from the mother's blood and carries it to all parts of the baby's body so they can keep growing.

Why doesn't the mother's blood go into the baby?
The mother's heart pumps the blood strongly. All parts of her body are strong enough to stand up to the pumping of the blood. The parts of the baby's body are much weaker and would be destroyed by the power of the mother's blood. The baby develops a heart quickly so it can pump its own blood. The baby's heart pumps less strongly than the mother's heart so it does not damage its body.