NCIt definition : The Menstrual Cycle involves regularly recurring hormonal changes and physiologic
endometrial changes during the reproductive period in human females, and some primates,
and culminates in partial sloughing of the endometrium (menstruation) in the absence
of fertilization. For most women, one cycle takes 28 days. (NCI);
Alternative definition : NCI-GLOSS: The monthly cycle of hormonal changes from the beginning of one menstrual
period to the beginning of the next.;