obstetrician vs specialist

obstetrician and specialist both are nouns.

Word	Noun	Adjective	Verb	Adverb
obstetrician	Yes	No	No	No
specialist	Yes	No	No	No

As nouns, specialist is a hypernym of obstetrician; that is, specialist is a word with a broader meaning than obstetrician:

Other hypernyms of obstetrician include adult male, man, medical specialist.

obstetrician (noun)	specialist (noun)
a physician specializing in obstetrics	an expert who is devoted to one occupation or branch of learning
	practices one branch of medicine