premature labor vs lying-in

premature labor and lying-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
premature labor	Yes	No	No	No
lying-in	Yes	No	No	No

As nouns, lying-in is a hypernym of premature labor; that is, lying-in is a word with a broader meaning than premature labor:

premature labor: labor beginning prior to the 37th week of gestation
lying-in: concluding state of pregnancy; from the onset of contractions to the birth of a child

Other hypernyms of premature labor include childbed, confinement, labor, labour, parturiency, travail.

premature labor (noun)	lying-in (noun)
labor beginning prior to the 37th week of gestation	concluding state of pregnancy; from the onset of contractions to the birth of a child