Birth weight can also indicate whether a baby is born prematurely. Babies born with a weight of less than 2,500 grams (about five and one-half pounds) are considered to have low birth weight. Generally, infants with a birth weight less than 2,500 grams are premature. However, babies can be premature but weigh more than 2,500 grams at birth. They can also be born at term and weigh less than 2,500 grams. There is no perfect system for categorizing infants, and so measurements, birth weight, and gestational age, are all used.