The average weight of a baby is around 3 kg at birth. Children who weigh much less than 3 kg at birth are called low birth-weight infants. These babies are: born to mothers who have had such complications as diabetes or anaemia in pregnancy, or to mothers with infections, or to under-nourished mothers.
Multiple pregnancies, such as twins, may also result in babies weighing less than the average. Certain genetic defects, such as Down's Syndrome, may also cause babies to be born with low birth-weight.