premature baby vs preemie

premature baby and preemie both are nouns.

Word	Noun	Adjective	Verb	Adverb
premature baby	Yes	No	No	No
preemie	Yes	No	No	No

As nouns, premature baby and preemie are synonyms defined as:

premature baby and preemie: an infant that is born prior to 37 weeks of gestation

Other synonyms of premature baby include premature infant, premie, preterm baby, preterm infant.

premature baby (noun)	preemie (noun)
an infant that is born prior to 37 weeks of gestation	an infant that is born prior to 37 weeks of gestation