Low birthweight is defined as less than 2500 grams or 5 1/2 pounds. It is also common to classify low birthweight births into moderately low birthweight (1500-2499 grams) and very low birthweight (less than 1500 grams or 3 1/3 pounds). These classifications are useful because they often correspond to clinical characteristics - increasing morbidities or illnesses with decreasing birthweight. Babies born too small are often born too soon. While the causes of low birthweight and preterm birth may be different in some cases, there is significant overlap within these populations of infants.