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This might seem hard to believe,
这听上去令人难以置信，
but right now, 300 million women across the planet are experiencing the same thing: a period.
但是现在全球近3亿的女性正在经历着同样的事情：一段特殊的时期。
The monthly menstrual cycle that leads to the period is a reality most women on Earth will go through in their lives.
这就是每月一次的月经期，是一个全球妇女在她们的一生中必然要面对的现实。
But why is this cycle so universal? And what makes it a cycle in the first place?
但为什么这个周期如此普遍呢？最开始的周期是怎样形成的呢？
Periods last anywhere between two and seven days, arising once within in a 28-day rotation.
月经会持续2-7天，每28天循环发生。
That whole system occurs on repeat, happening approximately 450 times during a woman's life.
月经现象会重复不断地出现，女人的一生大概会有450次的月经。
Behind the scenes are a series of hormonal controls
表象之下，这一切都是由人体激素进行调控的，
that fine tune the body's internal workings to make menstruation start or stop during those 28 days.
激素精密地控制着身体内部的运作，在这28天里控制经期的起始。
This inner machinery includes two ovaries stocked with thousands of tiny sacks called follicles
这套内部机制包含两个卵巢，卵巢内存有成千上万的微小袋子，叫做卵泡，
that each contain one oocyte, an unfertilized egg cell.
每个卵泡内都含有一个卵母细胞，即未受精卵细胞。
At puberty, ovaries hold over 400 thousand egg cells,
青春期时期，卵巢内含有超过40万的卵细胞，
but release only one each month, which results in pregnancy or a period.
但每个月只会释放出一个卵细胞，正是这个卵细胞决定着女性是受孕还是月经。
Here's how this cycle unfolds.
接下来我们来看看这个循环过程是如何产生的。
Each month beginning around puberty,
青春期的每个月初时候，
the hormone-producing pituitary gland in the brain starts releasing two substances into the blood:
大脑中的脑下垂体会分泌激素向血液中释放两种物质：
follicle stimulating hormone and luteinizing hormone.
促卵泡激素和黄体化激素。
When they reach the ovaries, they encourage the internal egg cells to grow and mature.
当它们到达卵巢后，会促进子卵巢内卵泡的发育和成熟。
The follicles respond by pumping out estrogen.
卵泡的成熟，会进一步促进雌性激素的分泌。
The egg cells grow and estrogen levels peak, inhibiting the production of FSH, and telling the pituitary to pump out more LH.
伴随着卵细胞的成长，雌性激素的分泌也将达到峰值，同时抑制促卵泡激素，促进垂体释放更多的黄体化激素。
That causes only the most mature egg cell from one of the ovaries to burst out of the follicle and through the ovary wall.
结果就是，只有其中一个卵巢中发育最为成熟的卵细胞，才能冲出卵泡，穿越子房壁。
This is called ovulation, and it usually happens ten to sixteen days before the start of a period.
这就是排卵现象，排卵通常发生在月经前的10到16天。
The tiny oocyte moves along the fallopian tube.
微小的卵母细胞沿输卵管运动。
A pregnancy can only occur if the egg is fertilized by a sperm cell within the next 24 hours.
只有当卵母细胞（卵子）和精子结合后女性才会受孕，这需要在排卵后24小时之内发生。
Otherwise, the egg's escapade ends, and the window for pregnancy closes for that month.
否则，意味着卵子使命的终结，女性在该月受孕的窗口也会关闭。
Meanwhile, the now empty follicle begins to release progesterone,
与此同时，空虚的卵泡开始释放孕酮，
another hormone that tells the womb's lining to plump up with blood and nutrients
另一种激素以刺激子宫内膜产生血液和营养物质，
in preparation for a fertilized egg that may embed there and grow.
为受精卵的着床和生长做准备。
If it doesn't embed, a few days later, the body's progesterone and estrogen levels plummet,
如果没有受精卵着床，几天后，身体的孕酮和雌性激素骤降，
meaning the womb stops padding out and starts to degenerate, eventually falling away.
刺激子宫停止补充养分，内膜开始退化坏死，渐渐脱落。
Blood and tissue leave the body, forming the period.
血液和脱落的子宫内膜离开身体，形成月经。
The womb can take up to a week to clear out its unused contents, after which, the cycle begins anew.
子宫清理内部无用物质的过程常常会持续一周，之后一个新的周期又开始了。
Soon afterwards, the ovaries begin to secrete estrogen again,
很快，卵巢又开始分泌雌性激素，
and the womb lining thickens, getting ready to accommodate a fertilized egg or be shed.
子宫内膜开始变厚，为受精卵着床最准备或再一次脱落。
Hormones continually control these activities by circulating in ideal amounts delivered at just the right time.
激素长期控制着整个活动过程，通过在精确的时间输送理想的量。
The cycle keeps on turning, transforming each day and each week into a milestone along its course towards pregnancy or a period.
随着周期的循环前行，令每一天，每个星期，都是这个受孕或经期过程中的一个里程碑。
Although this cycle appears to move by clockwork, there's room for variation.
虽然这个过程看上去如时钟般精准，但它仍然会有区别。
Women and their bodies are unique, after all.
毕竟每位女性及其身体都是独一无二的。
Menstrual cycles occur at diffferent times in the month,
每个月，月经产生的时间因人而异；
ovulation comes at various points in the cycle, and some periods last longer than others.
排卵期出现的节点不同；甚至经期持续的时间也各有长短。
Menstruation even begins and ends at different times in life for different women, too.
另外，对于不同的女性来说，一生中初潮和绝经的时间也不尽相同。
In other words, variations between periods are normal.
换言之，不同个体的经期有差别是正常的。
Appreciating these differences and learning about this monthly process can empower women,
欣赏这些不同并学习关于生理周期的过程，能让女性更加的从容自主，
giving them the tools to understand and take charge of their own bodies.
让女性了解并掌控自己的身体。
That way, they're able to factor this small cycle into a much larger cycle of life.
这样，她们能够将从这个小周期领会到的东西，运用在更加复杂的生活周期之中。
