preterm infant vs preterm baby

preterm infant and preterm baby both are nouns.

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

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

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

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

preterm 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