中英文稿

地球上只有極少數(shù)的物種 有這種神秘的特質(zhì)：?月經(jīng)周期。?人類是便是這少數(shù)物種之一。?猴子、猿、蝙蝠、人類， 可能還有象鼩?是地球上僅有的 有月經(jīng)的哺乳動物。?人類的月經(jīng)比其他動物更頻繁，?即便月經(jīng)會導(dǎo)致營養(yǎng)流失 和身體上的不便。?那么這種罕見的生理過程 究竟意義何在？?這要從懷孕說起。?在懷孕過程中， 身體的資源被充分地利用?來塑造一個適合胎兒生長的環(huán)境，?為胎兒成長提供營養(yǎng)的港灣。?從這方面來看， 懷孕多么神奇啊。?但這只是一方面。?另一方面，懷孕的過程中 母體和胎兒也存在沖突。?和其他所有生物一樣，?人類身體進(jìn)化的目的是 更好地延續(xù)物種基因。?對母親而言，這意味著 她應(yīng)該給她所有的后代?提供相同的條件。

?但母親和胎兒的基因 不是完全相同的。?胎兒還繼承了其父親的基因。?這些基因?yàn)榱松妫?會向母親索取更多的資源。?這種進(jìn)化中的利益沖突，?讓女性和她腹中的孩子?在子宮內(nèi)展開 一場生理上的拔河比賽。?這場比賽的成因之一是胎盤。?胎盤是連接胎兒和 母體血液系統(tǒng)的器官。?它為胎兒提供發(fā)育需要的營養(yǎng)。?大多數(shù)哺乳動物的胎盤 被母體內(nèi)的一層細(xì)胞屏障包圍。?這層屏障讓母體能控制 給胎兒的營養(yǎng)供應(yīng)。?但對于人類和其他少數(shù)物種來說，?胎盤實(shí)際上直接進(jìn)入了 母體的循環(huán)系統(tǒng)，?直接接觸母體的血液。?通過胎盤， 胎兒向母體的動脈釋放荷爾蒙，?這使母體的動脈擴(kuò)張， 運(yùn)送營養(yǎng)豐富的血液。?這樣無限制的接觸， 胎兒能通過控制激素?來增加母體血糖， 擴(kuò)張母體動脈，?并提高母體血壓。?

在必要的情況下，大多數(shù)哺乳動物的母體 能排出或者再吸收胚胎，?但是對人類來說， 一旦胎兒連接到血液供應(yīng)，?切斷了這種連接 會導(dǎo)致嚴(yán)重內(nèi)出血。?如果胎兒發(fā)育不良或死亡，?則會危及母親的健康。?胎兒發(fā)育和對營養(yǎng)的持續(xù)需求?會使母親極其疲憊，血壓升高，?并可能出現(xiàn) 糖尿病和先兆子癇等狀況。?因?yàn)檫@些風(fēng)險的存在，?懷孕是一項(xiàng)重大而危險的投資。?因此身體有理由仔細(xì)篩選胚胎，?只留下那些值得為之冒險的胚胎。?月經(jīng)則隨之產(chǎn)生了。?懷孕始于“著床”這一過程，?胚胎把自己嵌入到子宮內(nèi)膜上。?子宮內(nèi)膜不斷演化 使得著床變得困難。?這樣只有健康的胚胎能夠存活。?但這樣一來，?它也選擇了最有活力的胚胎，?創(chuàng)造了一個進(jìn)化反饋循環(huán)。?胚胎向子宮內(nèi)膜傳遞 復(fù)雜而精細(xì)的激素信號，?使子宮內(nèi)膜允許它著床。?那么如果胚胎著床失敗了呢？?胚胎可能還會附著在子宮內(nèi)膜上，?甚至部分進(jìn)入子宮內(nèi)膜。?但胚胎慢慢死去時， 母體會更容易發(fā)生感染。?它可能還會一直釋放激素信號， 擾亂母體組織。?身體為了避免這個問題， 會排出所有可能的危險。?每當(dāng)排卵未能形成健康胚胎，?子宮都會使整個內(nèi)膜脫落，?同時將未受精的，虛弱的 或?yàn)l死的胚胎一并排出。?這個自我保護(hù)的過程 就是月經(jīng)周期，?其結(jié)果就是月經(jīng)來潮。?正是這種奇異的生物特性?使得人類能一直繁衍下去。

A handful of species on Earth share a seemingly mysterious trait:?a menstrual cycle.?We're one of the select few.?Monkeys, apes, bats, humans, and possibly elephant shrews?are the only mammals on Earth that menstruate.?We also do it more than any other animal,?even though its a waste of nutrients and can be a physical inconvenience.?So where's the sense in this uncommon biological process??The answer begins with pregnancy.?During this process, the body's resources are cleverly used to shape?a suitable environment for a fetus,?creating an internal haven for a mother to nurture her growing child.?In this respect, pregnancy is awe-inspiring,?but that's only half the story.?The other half reveals that pregnancy places a mother and her child at odds.?As for all living creatures,?the human body evolved to promote the spread of its genes.?For the mother, that means she should try to provide equally?for all her offspring.?But a mother and her fetus don't share exactly the same genes.?The fetus inherits genes from its father, as well,?and those genes can promote their own survival by extracting?more than their fair share of resources from the mother.?This evolutionary conflict of interests?places a woman and her unborn child in a biological tug-of-war?that plays out inside the womb.?One factor contributing to this internal tussle?is the placenta, the fetal organ that connects to the mother's blood supply?and nourishes the fetus while it grows.?In most mammals, the placenta is confined behind a barrier of maternal cells.?This barrier lets the mother control the supply of nutrients to the fetus.?But in humans and a few other species,?the placenta actually penetrates right into the mother's circulatory system?to directly access her blood stream.?Through its placenta, the fetus pumps the mother's arteries with hormones?that keep them open to provide a permanent flow of nutrient-rich blood.?A fetus with such unrestricted access can manufacture hormones?to increase the mother's blood sugar, dilate her arteries,?and inflate her blood pressure.?Most mammal mothers can expel or reabsorb embryos if required,?but in humans, once the fetus is connected to the blood supply,?severing that connection can result in hemorrhage.?If the fetus develops poorly or dies,?the mother's health is endangered.?As it grows, a fetus's ongoing need for resources can cause intense fatigue,?high blood pressure,?and conditions like diabetes and preeclampsia.?Because of these risks,?pregnancy is always a huge, and sometimes dangerous, investment.?So it makes sense that the body should screen embryos carefully?to find out which ones are worth the challenge.?This is where menstruation fits in.?Pregnancy starts with a process called implantation,?where the embryo embeds itself in the endometrium that lines the uterus.?The endometrium evolved to make implantation difficult?so that only the healthy embryos could survive.?But in doing so,?it also selected for the most vigorously invasive embryos,?creating an evolutionary feedback loop.?The embryo engages in a complex, exquisitely timed hormonal dialogue?that transforms the endometrium to allow implantation.?What happens when an embryo fails the test??It might still manage to attach,?or even get partly through the endometrium.?As it slowly dies, it could leave its mother vulnerable to infection,?and all the time, it may be emitting hormonal signals that disrupt her tissues.?The body avoids this problem by simply removing every possible risk.?Each time ovulation doesn't result in a healthy pregnancy,?the womb gets rid of its endometrial lining,?along with any unfertilized eggs, sick, dying, or dead embryos.?That protective process is known as menstruation,?leading to the period.?This biological trait, bizarre as it may be,?sets us on course for the continuation of the human race.