premature baby vs preterm infant

premature baby and preterm infant both are nouns.

Word	Noun	Adjective	Verb	Adverb
premature baby	Yes	No	No	No
preterm infant	Yes	No	No	No

As nouns, premature baby and preterm infant are synonyms defined as:

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

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

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