Fetal growth Life begins when the egg and sperm combine to form a cell called the fertilized egg. Examine the entire process from fertilized egg to fetus. Fetus at 26-day-old At the end of the first month of pregnancy, the fetus has a rounded head, arched back, and a long tailed vertebrate. The fertilized egg, which was a single cell, split into hundreds of cells in just four weeks, and developed into organized cells with major human strains such as nerves, muscles, blood vessels, and skeleton. At this time, the heart, brain, spinal cord, and sensory organs begin to form and function to some extent. It is still embryonic and small, C-shaped. Fetus at 6 weeks Beginning at 4 weeks, there are visible small spots on the face, and at 6 weeks, the first eyelid is formed, taking the initial form of the retina. You can start the heartbeat and hear the beats by ultrasound, but the two vessels are tubular because they don't have heart shape yet. When the head grows to become second c...