WordNet records no synonym and no antonym for menstrual cycle. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
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
“the average menstrual cycle is 28 days”
Synonyms, antonyms and the broader and narrower relations for menstrual cycle come from Princeton University’s WordNet. Full sources and licences.