Menstrual cycle
The menstrual cycle isperiodic change inwoman's body that occurs every month between pubertymenopauseis relatedreproduction. The average human menstrual cycle28 days long, butcan range from 2135 days. Itcontrolled by hormones.
| Tablecontents |
|
2 Timing 3 Hormonal control 4 Terminology,menstruationother animals |
The cycle
By convention,onsetmenstrual bleeding (menstruation or menses) marksbeginning ofcycle. Menstruation lastsseveral daysis caused byloss oflining ofuterus. The uterus was readyacceptfertilized ovum (egg), but none arrived,solining (called endometrium)expelled. Therefore, if menstrual bleeding occurs,woman knows that shenot pregnant. During menstruation, women typically use tampons (plugs made from absorbent materialinserted intovagina) or sanitary towels (worn outsidevagina)preventsoilingclothes.
Thennew egg matures inovaries,about atmiddle ofcycle (14 days before beginning ofnext menstrual bleeding), ovulation occurs, meaning thateggreleased byovaryentersfallopian tube. In some women, ovulationaccompanied bycharacteristic pain called Mittelschmerz which lastsseveral hours. A characteristic clearstringy mucus develops atcervix, readyaccept sperm. The egg (withdiameterabout 0.5 mm) travels throughfallopian tube touterus, pushed along by movements oflining oftube. This trip takes about one day, andeggavailablebe fertilized during this period.
Inmeantime,endometrium has startedgrow again. If fertilization occurs,egg implants inwall ofuterusmajor changes take place, withmenstrual cycle being suspended forlength ofpregnancy. If no fertilization occurs,endometriumlostbleeding andcycle starts again.
About 50 millilitresbloodlost during menstruation. The bloodprevented from clotting by an enzyme called plasmin contained inendometrium. In most women, menstruationpreceded or accompanied by various unpleasant symptoms caused byinvolved hormonesby cramping ofuterus. Among theseabdominal pain, migraine headache, depressionirritability. Premenstrual stress syndrome (PMS or premenstrual syndrome) can also occur.
All ofwoman's eggspresent inovaries attimebirth. An estimated 250,000400,000 immature eggs existeach ovary. Onlysmall fractionthem ever develop into mature eggs.
Timing
Whilelength offirst part ofcycle (from onsetbleedingovulation) varies among women,second part (from ovulationonsetbleeding) almost always14 days long.
Sperm can live3-4 days (possibly up7 days) insidewoman, somost fertile period (withhighest likelihoodsexual intercourse leadingpregnancy) istime some 5 days before until 1-2 days after ovulation. Innormal four-week cycle, this corresponds tosecond andbeginning ofthird week ofcycle. (Itimportantnote that pregnancy can occur from intercourse at any time duringmenstrual cycle, even during menstruation.) Various natural family planning methodsbirth control attemptdetermineprecise timeovulationorderfindfertileinfertile days incycle.
Hormonal control
The main hormones involvedcontrol ofmenstrual cycleoestrogenprogesterone. Atbeginning ofmenstrual cycle,pituitary gland releases Follicle Stimulating Hormone (FSH), signalingimmature folliclesgrow inovaries. The follicle issac containingegg. Oestrogen levels rise ashormonesecreted bydeveloping follicles. During ovulation,follicle andovary's wall burst, releasingegg; oestrogen levelsmaximal. After ovulation, both oestrogenprogesteronesecreted bycorpus luteum which developed fromburst follicleremained inovary. The purposeprogesterone ispreparebody forpossible pregnancy. Oncecorpus luteum dies, hormone levels fall which causesejection ofendometriummenstruation. Oestrogenprogesteronealsomain ingredientsmost birth control pills.
Even though theretwo ovaries, normally only one egg will be produced per period. Which ovary "wins"essentially random; thereno left/right coordination involved. Aslevel offollicle stimulating hormone (FSH) increases,stimulatesproduction offollicle,follicle secretes inhibin, which shuts offFSH, preventing more follicles from developing. So each month,depends on whetherleft or right ovary islucky oneproducefirst follicle.
Terminology,menstruationother animals
The terms "menstruation""menses" come fromLatin mensis (month) from Greek mene (moon)reference tofact thatlunar monthalso approximately 28 days long. Thereno connection between lunar monthsmenstrual periods however, asshown byfact thatgreat apes have menstrual cycles that vary from 29 daysorangutans37 dayschimpanzees.
A regular menstrual cycle as described here only occurs inhigher primates. Femalesother mammalian species go through certain episodes called "estrus" or "heat"each breeding season. During these times, ovulation occursfemalesreceptivemating,fact advertisedmalessome way. If no fertilization takes place,endometriumreabsorbed byuterusno menstrual bleeding occurs.
