It all started with one cell from your mother and one from your father. These cells came together and made one (fertilised) cell. (See How you were made).
Cell division
The fertilised cell moved to the womb, splitting itself a number of times. One cell became two, then the two became four, and so on. Cell division is the principle of life. Every living thing in the world consists of cells which can divide. Simple life forms, like bacteria, consist of one cell only.
Humans consist of many different kinds of cells. Hair cells are different from skin cells and muscle cells differ from fat cells. But all these cells that make you up started with the one fertilised cell.
Embryo
About seven days after fertilisation you entered the womb and settled in its wall. About eight cell divisions had taken place and you consisted of a few hundred cells, all together still very small. Smaller than the head of a pin. Cell division continued and in a few weeks you grew to a few millimeters in length. You were now an embryo and you looked exactly like the embryos of other animals. The cells of the embryo were already different from each other. Some cells would become bone cells, others would construct your heart and lungs, your nails and skin, your hair and your nose. You had no say in who you are.
Foetus
After two months you had grown to a length of several centimetres and were now called ‘foetus’. After four months it was clear from your sex organs that you would become a girl or a boy. After six months you were already beginning to look like a baby. You weighed about a thousand gram, but you were not yet ready to be born. Your lungs were not completed yet, and everything was still very vulnerable. Not until three months later were you ready for birth. (Read also How you came into the world).