Title: How A Fetus Grows | HowStuffWorks
Headings: How A Fetus Grows
How A Fetus Grows
How A Fetus Develops in the First Trimester
How A Fetus Develops in the Second Trimester
How A Fetus Develops in the Third Trimester
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Content: Publications International, Ltd.
In the first weeks after conception, the single cell rapidly divides into many cells. A hollow ball of cells is formed and becomes attached to the wall of the uterus (womb). Some of the cells become the placenta; the rest become the embryo. The latter group of cells develops into a four-layered disk. Each layer is converted into different areas of the body. The outer layer, or ectoderm, for example, develops into skin, hair, nails, and the nervous system. The inner layer, or endoderm, develops into the intestines and lungs. The middle layers develop into the heart, bones, and muscles. By three weeks after fertilization, or about one week after the first period is missed, the embryo is 1/10 inch long and has an oval shape.