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
es 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 pathogen
s 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.