low-birth-weight baby has 1 distinct synonym spread across 1 of its 1 sense. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an infant born weighing less than 5.5 pounds (2500 grams) regardless of gestational age
“a low-birth-weight infant is at risk for developing lack of oxygen during labor”
Synonyms, antonyms and the broader and narrower relations for low-birth-weight baby come from Princeton University’s WordNet. Full sources and licences.