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: 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. In the next few weeks, it becomes more curved, and a head and tail become discernible. The beginnings of the spinal cord and brain take shape. A heart begins to form. Tiny eyes can be seen. Arms and legs begin to bud.