A fetus (also foetus) is a developing mammal after the embryonic stage and before birth. The plural is fetuses or foetuses. In humans, a fetus develops from the end of the 8th week of pregnancy (when the major structures have formed), until birth. Factors affecting fetal growth can be maternal, placental, or fetal. Maternal factors include maternal size, weight, weight for height, nutritional state, anemia, high environmental noise exposure, cigarette smoking, substance abuse, or uterine blood flow. Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization and nutrient production. Fetal factors include the fetus genome, nutrient production, and hormone output. Inappropriate growth can result in low birth weight.