In humans, the fetal stage commences at the beginning of the ninth week.[1] At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown to rump, and weighs about 8 grams.[1] The head makes up nearly half of the fetus' size.[9] Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.[10] The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.[11][12] The genitalia of the fetus starts to form and placenta becomes fully functional during week 9.[13]