A fetus or foetus is that 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 fertilization (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 main body organs, though they're going to not yet be fully developed and functional and a few not yet situated in their final anatomical location. There is much variation within the growth of the human fetus. When the fetal size is less than expected, the condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be maternal, placental, or fetal.