safe period vs menstrual cycle

safe period and menstrual cycle both are nouns.

Word	Noun	Adjective	Verb	Adverb
safe period	Yes	No	No	No
menstrual cycle	Yes	No	No	No

safe period (noun)	menstrual cycle (noun)
that time during a woman's menstrual cycle during which conception is least likely to occur (usually immediately before of after menstruation)	a recurring cycle (beginning at menarche and ending at menopause) in which the endometrial lining of the uterus prepares for pregnancy; if pregnancy does not occur the lining is shed at menstruation