SGA infant vs newborn

SGA infant and newborn both are nouns.

Word	Noun	Adjective	Verb	Adverb
SGA infant	Yes	No	No	No
newborn	Yes	Yes	No	No

As nouns, newborn is a hypernym of SGA infant; that is, newborn is a word with a broader meaning than SGA infant:

SGA infant: an infant whose size and weight are considerably less than the average for babies of the same age
newborn: a baby from birth to four weeks

Other hypernyms of SGA infant include neonate.

SGA infant (noun)	newborn (noun)
an infant whose size and weight are considerably less than the average for babies of the same age	a baby from birth to four weeks

SGA infant (adjective)	newborn (adjective)
	having just or recently arisen or come into existence
	recently born