We're one of the select few.

我們就是這少數的其中之一。

Monkeys, apes, bats, humans, and possibly elephant shrews are the only mammals on Earth that menstruate.

猴子、人猿、蝙蝠、人類， 可能也包括象鼩是地球上僅有會月經來潮的哺乳類。

We also do it more than any other animal, even though it's a waste of nutrients and can be a physical inconvenience.

而我們人類的月經來得比其他動物更頻繁，雖然這會耗費營養，亦可能造成身體不便。

So where's the sense in this uncommon biological process?

那麼這不尋常的生物過程有甚麼意義？

The answer begins with pregnancy.

要解答，需先認識懷孕過程。

During this process, the body's resources are cleverly used to shape a suitable environment for a fetus,

在這個過程中， 身體會巧妙地運用資源來營造一個適合胎兒的環境，

creating an internal haven for a mother to nurture her growing child.

為母親創造一個孕育成長胎兒的內在庇護所。

In this respect, pregnancy is awe-inspiring, but that's only half the story.

從這方面來看， 懷孕是件令人驚嘆的事情，但這只是故事的一半。

The other half reveals that pregnancy places a mother and her child at odds.

故事的另一半揭示懷孕使母親和胎兒處於衝突狀態。

As for all living creatures, the human body evolved to promote the spread of its genes.

正如所有生物人體演化以促進其基因的延續。

For the mother, that means she should try to provide equally for all her offspring.

對於母親而言，這意謂她應該提供相等的養育條件來給予她所有的後代。

But a mother and her fetus don't share exactly the same genes.

但母親和胎兒並非擁有完全相同的基因。

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.

胎兒的基因也同時遺傳自其父親，而那些基因為了提高它們的存活率會從母體吸取多於他們應得的營養。

This evolutionary conflict of interests places a woman and her unborn child in a biological tug-of-war that plays out inside the womb.

這種演化而來的利益衝突讓母親和她的胎兒處於一場生物拉鋸戰，就在子宮內上演。

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.

這場內在戰爭的原因之一是胎盤，胎兒透過這器官來連結母親的血液並供應胎兒成長時的營養。

In most mammals, the placenta is confined behind a barrier of maternal cells.

大多數哺乳類動物的胎盤被母親體內的一層細胞屏障圍著。

This barrier lets the mother control the supply of nutrients to the fetus.

這屏障讓母親得以控制胎兒營養的供給。

But in humans and a few other species, the placenta actually penetrates right into the mother's circulatory system to directly access her bloodstream.

但在人類和少數其他動物胎盤卻是直接連結於母親的循環系統，直接接觸她的血流。

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.

經由胎盤，胎兒在母親的動脈泵送數種荷爾蒙讓血管保持通暢，持續提供富含養分的血流。

A fetus with such unrestricted access can manufacture hormones to increase the mother's blood sugar, dilate her arteries, and inflate her blood pressure.

在這種不受限的接觸下， 胎兒能製造荷爾蒙使母親血糖升高、動脈擴張和血壓上升。

Most mammal mothers can expel or reabsorb embryos if required, but in humans, once the fetus is connected to the blood supply,

大多數哺乳類母親在必要時能排出或吸收胚胎，但因為人類的胎兒 一旦連結至母親的血流供應後，

severing that connection can result in hemorrhage.

切斷這連結可能會導致出血。

If the fetus develops poorly or dies, the mother's health is endangered.

如果胎兒發育不良或死亡將會危及母親的健康。

As it grows, a fetus's ongoing need for resources can cause intense fatigue, high blood pressure, and conditions like diabetes and preeclampsia.

當胎兒日漸長大，其持續的營養需求， 會使母親非常疲倦、高血壓及一些狀況，例如糖尿病和子癇前症。

Because of these risks, pregnancy is always a huge, and sometimes dangerous, investment.

因為這些風險，懷孕總是個重大而時有危險的投資。

So it makes sense that the body should screen embryos carefully to find out which ones are worth the challenge.

所以不難明白， 身體應要小心地篩檢胚胎，找出那些胚胎是值得冒險的。

This is where menstruation fits in.

月經剛好符合這份工作。

Pregnancy starts with a process called implantation, where the embryo embeds itself in the endometrium that lines the uterus.

懷孕始於「著床」，即是胚胎自行植入子宫内膜。

The endometrium evolved to make implantation difficult, so that only the healthy embryos could survive.

子宫内膜經過演化， 使得著床難以進行，因此，只有健康的胚胎能夠生存。

But in doing so, it also selected for the most vigorously invasive embryos, creating an evolutionary feedback loop.

但這樣同時也挑選出植入力最佳的胚胎，從而演化出一個回饋機制。

The embryo engages in a complex, exquisitely timed hormonal dialogue that transforms the endometrium to allow implantation.

透過複雜而調節精細的荷爾蒙機制改變子宮內膜的形態， 讓胚胎著床得以進行。

What happens when an embryo fails the test?

如果胚胎未能通過考驗， 又會怎樣呢？

It might still manage to attach, or even get partly through the endometrium.

它或許仍然能夠附著或甚至部分植入在子宮內膜。

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.

當它漸漸死去， 它可能使母親易受感染在這個過程中，它或會釋放出擾亂母親身體組織的荷爾蒙訊息。

The body avoids this problem by simply removing every possible risk.

身體為了避免這種情況， 會盡可能排除種種風險。

Each time ovulation doesn't result in a healthy pregnancy, the womb gets rid of its endometrial lining,

每當排卵未能發展成健康的懷孕，子宮內膜的表層會崩塌剝落，

along with any unfertilized eggs, sick, dying, or dead embryos.

未受精卵，不健康、瀕死或已死的胚胎也一併被排出體外。

That protective process is known as menstruation, leading to the period.

這個保護母親身體的過程就是月經來潮乃至有月經週期。

This biological trait, bizarre as it may be, sets us on course for the continuation of the human race.

這生物過程或許奇怪，但正是它讓我們得以繁衍子孫，綿延後代。