premature baby vs premie

premature baby and premie both are nouns.

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

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

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

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

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