The fetal stage commences at the beginning of the 9th week. At the start of the fetal stage, the fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and weighs about 8 grams. The head makes up nearly half of the fetus' size. Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.