Ask A Scientist© Biology Archive

Question:  What is the menstrual cycle?
Sue C Robinson

Answer:  Being pregnant is tough.  You've got to eat for two, you can't run as
fast and you're not too agile. In a hostile environment the pregnant female
runs the risk of being eaten or dying of starvation.  So the idea is you
get it over with *as fast as possible*.  Once the egg of a female gets
fertilized, it grows as fast as it can.  By the time it travels from the
ovary down to the uterus it already needs a place to sit and grow and
receive nourishment.  It grows a placenta, basically an external intestine
that snuggles up against the uterus to suck up food and oxygen from the
mother's blood.  To help this out the mother grows an endometrium, or
lining of the uterus, that is rich in blood vessels and easy to snuggle up
to.  But this lining takes weeks to grow, and you don't want that egg
waiting around, as I said.  So what happens is every month the lining grows
*ahead of time* and waits to see if the egg comes down fertilized.  If it
does, great.  If not, the lining just dies and falls off and you start
again.  The lining is discharged from the vagina as a mess of blood and
dead cells called menstruum or menstrual flow.  The whole cycle of grow
lining, wait, drop it off is called the menstrual cycle.  The cycle  begins
at menarche, age 13 +/- a few years, and ceases at menopause, age 50 +/-.
All these "men" words come from the Latin "menses" which means "month"
because the cycle restarts every month.  "Month" (and "menses") come from
"Moon" because the Moon goes around the Earth in a month.  Why does the
cycle take the same time as it takes the Moon to go around the Earth?   No
one knows. It's one of the world's interesting mysteries.  All healthy
normal adult women have menstrual cycles, and many suffer cramps during it.
There are medicines (ibuprofen) that can help bad cramps.
Christopher Grayce