Low birthweight is when a baby is born weighing less than 5 pounds, 8 ounces. Low birth weight (LBW) is the medical classification for a baby who weighs less than 2500 grams—or 5 lbs 5 oz—at birth. Low birth weight (LBW) is defined by the World Health Organization as a birth weight of a infant of 2,499 g (5 lb 8. Low birth weight is a term used to describe babies who are born weighing less than 5. Low birth weight is a result of preterm birth (PTB, short gestation <37 completed weeks), intrauterine. Optimal feeding of low-birth-weight infant contains recommendations on what to feed, when to feed and how to feed a LBW newborn.