premature infant vs preterm baby

premature infant and preterm baby both are nouns.

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

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

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

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

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