menstrual cycle vs safe period

menstrual cycle and safe period both are nouns.

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

menstrual cycle (noun)	safe period (noun)
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	that time during a woman's menstrual cycle during which conception is least likely to occur (usually immediately before of after menstruation)