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During pregnancy, a single cell grows and develops into a human being able to have an independent life. This wonderful procedure takes about nine months and is known as the prenatal development. The baby’s development is grouped into three stages, called periods of the zygote, the embryo and the fetus. The first stage in the development is the period of the zygote or fertilized egg, which lasts for about two weeks and is the size of a pinhead. In this period, the zygote travels downwards to the Fallopian tube and the egg attaches to the lining of the uterus, which gives a soft, warm bed. From the uterus the zygote takes nourishment from the mother’s body. The zygote grows by cell division. The single complete cell divides and becomes two, then four and so on until there is a mass of cells. The second stage is known as the period of the embryo, which lasts from about the third through the eighth week of pregnancy. At this time the embryo grows very fast. First the mass of cells develops into all the major systems of the human body such as the heart, lungs, bones and muscle.