A human being's heart is about the size of that human being's fist. As the body develops, the heart grows at the same rate as the fist. So an infant's heart and fist are about the same size at birth. In the womb, however, that similarity was not always true. During the first few weeks after conception, the fetal heart occupies most of the fetus' mid-section. The heartsize to bodysize ratio is nine times greater in the fetus than in the infant. During those first few weeks, the fetal heart lies high in the chest. Soon, it moves down to occupy its position in the chest cavity. There are several phases of the fetal heart's development. At first, the heart is just a tube. It grows so fast that it needs more space, so it bends and twists back, forming the familiar shape. During the next phase, the two atria are partly separate but there is just one big ventricle. The next phase begins when the two atria are completely separate and the ventricles are just beginning to separate. Finally, the ventricles separate completely and the heart is developed. During the fetal heart's developmental stages, the heart actually takes on several distinct appearances. These heart structures resemble other animal hearts. During phase one, the tube-like heart is much like a fish heart. The second phase, with two chambers, resembles a frog heart. The three-chambered phase is similar to a snake or turtle heart. The final four-chambered heart structure distinguishes the human heart. View the structure of a preserved heart here. The heart, like other body parts, needs oxygen in order to grow and develop properly. During childhood, the body's years of rapid growth, the need for oxygen is greatest. The heart's rate of pumping oxygen-rich blood is fastest in infancy, about 120 beats per minute. As the child grows, the heart rate slows. A seven year old child's heart beats about 90 times per minute. By the age of 18, the heart rate has stabilized to about 70 beats per minute. By adulthood, the heart is fully developed. Throughout life, the heart needs only to be maintained and kept healthy in order to function. If you take care of your heart, by following a prescription for a healthy heart, your heart should take care of you for the rest of your life.