How did my life begin?
All living things are made up of cells. They are so small that you need a microscope to see them. Your body contains trillions and trillions of cells. Each person begins life as a single fertilized cell. This single cell contains all the information needed for a new human being to grow and live. The information—coded chemical instructions known as genes—is found on 23 pairs of chromosomes in the nucleus, or control center, of the cell. That special fertilized cell began with a single egg cell from your mother. Each month a woman releases a mature, or ripe, egg cell from reproductive organs called ovaries. This egg contains half the genes needed to create a new life. A man produces millions of sperm cells in reproductive organs called testes. Each sperm cell contains half the genes needed to create a new life. When a sperm cell from your father joined with and fertilized the released egg cell inside your mother’s body, the cell that would become you was complete. It had all the coded instructions it needed to begin dividing and growing into a baby. Within a few hours, the fertilized cell split into two complete cells, each with a full set of genes inside. Before long the cells divided again. After five or six days a ball of hundreds of cells existed. The size of the head of a pin, this ball of cells attached to the lining of your mother’s uterus, or womb, the reproductive organ in which babies grow. There, in the nourishing lining of the uterus, the cells continued to multiply. Gradually the cells began to specialize, turning into nerve cells, muscle cells, and so on. A tiny baby began to take shape. As you grew, you received nutrients and oxygen from your mother’s blood through a special tube that was attached to your abdomen called the umbilical cord. After 40 weeks (between nine and ten months), all of your organs and body systems were developed enough to work on their own, and you were ready to enter the world. Then you made your grand entrance and were born!