puerperium vs period of time

puerperium and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
puerperium	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of puerperium; that is, period of time is a word with a broader meaning than puerperium:

puerperium: time period following childbirth when the mother's uterus shrinks and the other functional and anatomic changes of pregnancy are resolved
period of time: an amount of time

Other hypernyms of puerperium include period, time period.

puerperium (noun)	period of time (noun)
time period following childbirth when the mother's uterus shrinks and the other functional and anatomic changes of pregnancy are resolved	an amount of time