A fetus or foetus (/ˈfiːtəs/; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilisation (or eleventh week gestational age) and continues until birth. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location. In humans, the fetal stage starts nine weeks after fertilization. At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown-rump, and weighs about 8 grams.The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the 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. The genitalia of the fetus starts to form and placenta becomes fully functional during week 9.