Menstruation is usually thought of as the
periodic discharge through the
vagina of a bloody
secretion containing
tissue debris from the shedding of the
endometrium from the nonpregnant
uterus.
However, as the
average frequency of
human intercourse, every three days or so, is high enough to make it likely that an
ovulation will result in a
pregnancy, this continuous sexual activity could also mean that
bacteria and
viruses can hitch regular free rides deep into the woman's reproductive tract.
One defense against such
infection is the plug of
mucus at the
cervix that blocks
sperm from ascending except during two or three fertile days a month. Another natural defense mechanism is
menstruation. The hot flow of blood conbined with the change in
pH is disastrous for
pathogens and it sweeps them away from the sweet
environment of the woman's innards.
In the natural environment, most women would experience far fewer menstrual cycles, since they would not cycle while pregnant or lactating, which would be most of time, given the standard of cyclical estrous and birth rates among mammals. Since humans do have sex more frequently, but have fewer offspring, menstruation is an important evolutionary defense mechanism designed to keep the females of the human race healthy.