The WHO definition of low (<2500g) and very low (<1500g) is referenced in many academic papers, but I haven't seen a citation for it. Is there a WHO document I can cite with these definitions?
A low birth weight baby is one that weighs less than 2500 g at birth. As growth is a progressive process, a baby may weigh less than 2500 g at birth because it is born too soon, or because it is small for its gestational age.
LBW infants are classified as very low birth weight (VLBW) if their birth weight is less than 1.5 kg, and as extremely low birth weight (ELBW) if their birth weight is less than 1 kg.
These are not necessarily the original sources of those classifications - neither one is cited, so it's not even necessarily possible to follow that definition backwards along a citation path, but those are both official WHO documents.