First Trimester

The first 3 months of fetal development are in many ways the most important. During this time, all the major organs in the body are formed. The embryo is particularly sensitive to assaults from the outside. By the end of this period the baby is not more than 3 inches long and weighs little more than 1 ounce. The time from fertilization to implantation in the uterus is about 5 to 7 days. After burrowing deep within the uterus, the egg begins to grow, doubling in size every day. The placenta has begun to form. In another week, the rudiments of a spinal cord are evident and, within days, five to eight vertebrae are in place. In addition, the eyes and heart have begun to form. Over the next few weeks the components of a human being develop, although at first the human baby is similar in appearance to the developing babies of some other mammals. The heart begins to form, as does the intestinal tract. At the end of the sixth week the brain becomes more noticeable, and arm and leg buds begin to appear. By the seventh week, the chest and abdomen are fully formed and the lungs are beginning to develop. The baby’s face and features are forming in the eight gestational week. Fingers and toes are beginning to develop. At the end of the second month of pregnancy, the baby looks like a human infant, albeit in miniature. By the tenth week, the baby’s face is well developed. The heart has four chambers and beats 120 to 160 beats per minute. At this point, the embryo is considered a fetus.