RN vs nurse-midwife

RN and nurse-midwife both are nouns.

Word	Noun	Adjective	Verb	Adverb
RN	Yes	No	No	No
nurse-midwife	Yes	No	No	No

As nouns, nurse-midwife is a hyponym of RN; that is, nurse-midwife is a word with a more specific, narrower meaning than RN:

RN: a graduate nurse who has passed examinations for registration
nurse-midwife: a registered nurse who has received special training as a midwife

Other hyponyms of RN include NP, nurse clinician, nurse practitioner.

RN (noun)	nurse-midwife (noun)
a graduate nurse who has passed examinations for registration	a registered nurse who has received special training as a midwife