Once a female reaches puberty, the uterus sheds an unfertilized egg along with the

If the egg is not fertilized, the lining of the uterus begin to pull away with the unfertilized egg.
if the egg is not fertilized in the days following its passage through the tube, the progesterone begins, too, to fall. The unfertilized egg eventually dissolve and the uterine wall surplus stands. The assembly is then discharged as "blood" to the outside, through the cervix and vagina.