Model released. For editorial use only. Comparison between a normal two day old baby (right, 3730g) with two low birthweight babies. The baby at left, aged four weeks, was born nine weeks before the term; the baby at centre, aged two weeks, was born at full term but weighed only 1200g. The average full term infant weighs 3400g at birth and few babies weigh less than 2500g or more than 4500g. Babies who weigh less than 2500g are classified of low birthweight. About 50% of these babies are small due to a premature birth, that is before the 37th week of pregnancy. Others are small because they are undernourished in the uterus due to an insufficient placenta.