Question re: evolution and menstruation

[Bob Campbell]

Female mammals exhibit two types of reprodutive cycles, i.e., estrous cycles
or menstrual cycles.  Menstrual cycles are characteristic of primates, and
are not unique to humans.  The length of menstrual cycles varies between
species.  Humans and rhesus monkeys both exhibit cycles of about 28 days.
Despite these general similarities, there are significant differences across
primates in their reproductive physiology, particularly in the implantation
of the embryo into the uterine lining and the development of the
maternal-fetal interface.

Hope this is helpful.

Bob Campbell