In terms of appearance, the stomachs of adults differ from those of children mainly in size. A baby that is only a day old has a stomach the size of a hazelnut, while a fully grown human being's stomach is the size of a grapefruit.
The human stomach keeps on growing until the person becomes an adult. By the second day after birth, a baby's stomach has grown to the size of a cherry. By day three, the baby's stomach is already the size of a walnut. It is important to keep on feeding an infant to allow the stomach to continually expand. Around the end of the first week of life, the baby will have significantly grown because his stomach is already as big as an apricot. By day 10, the stomach is the size of a large chicken egg, and its growth then slows.