preterm baby vs preemie

preterm baby and preemie both are nouns.

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

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

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

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

preterm 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