The monthly menstrual cycle that leads to the period is a reality most women on Earth will go through in their lives.

這個每月一次的循環就是所謂的月經，是地球上所有女性一生中都必須經歷的。

But why is this cycle so universal? And what makes it a cycle in the first place?

但是為什麼這個循環是如此的普遍呢？最初這個循環又是哪來的呢？

Periods last anywhere between two and seven days, arising once within in a 28-day rotation.

月經都會持續 2 至 7 天，每 28 天出現一個循環。

That whole system occurs on repeat, happening approximately 450 times during a woman's life.

整個系統不停地重複發生，一個女人生命中大概都要經歷 450 次。

Behind the scenes are a series of hormonal controls that fine-tune the body's internal workings to make menstruation start or stop during those 28 days.

在其之後是一連串的激素控制，其在細微地調整身體的內部運作，並使月經在 28 天內開始或暫停。

This inner machinery includes two ovaries stocked with thousands of tiny sacks called follicles that each contain one oocyte, an unfertilized egg cell.

這內部結構包含了兩個卵巢內積有數以千萬計的小囊，叫做卵巢濾泡，每一個裡面都含有一個卵母細胞，一個未受精的卵細胞。

At puberty, ovaries hold over 400 thousand egg cells but release only one each month, which results in pregnancy or a period.

在青春期的時候，卵巢內有超過 40 萬的卵細胞，但是每個月只排出一個， 此致懷孕或月經。

Here's how this cycle unfolds.

以下是這個循環的樣子。

Each month beginning around puberty, the hormone-producing pituitary gland in the brain starts releasing two substances into the blood: follicle-stimulating hormone and luteinizing hormone.

在青春期左右的時候開始這每月的循環，在大腦中產生賀爾蒙的腦下垂體開始在血液中排出兩種物質： 卵巢濾泡刺激賀爾蒙和黃體激素。

When they reach the ovaries, they encourage the internal egg cells to grow and mature.

當它們到達卵巢的時候，它們刺激內部卵細胞成長和熟成。

The follicles respond by pumping out estrogen.

卵巢藉由排出雌激素來反應。

The egg cells grow and estrogen levels peak, inhibiting the production of FSH, and telling the pituitary to pump out more LH.

卵細胞成長，且雌激素濃度提高，阻礙促濾泡成熟激素的生成，並且通知腦垂體要釋放更多黃體激素。

That causes only the most mature egg cell from one of the ovaries to burst out of the follicle and through the ovary wall.

這導致了只有其中一個卵巢中最成熟的卵細胞噴出濾泡，並經過卵巢壁。

This is called ovulation, and it usually happens ten to sixteen days before the start of a period.

這就稱為排卵，這通常在月經開始前 10 到 15 天會發生。

The tiny oocyte moves along the fallopian tube.

小小的卵細胞隨著輸卵管移動。

A pregnancy can only occur if the egg is fertilized by a sperm cell within the next 24 hours; otherwise, the egg's escapade ends, and the window for pregnancy closes for that month.

在接下來的 24 小時內，卵子與精子受精的時候就會懷孕，否則，意味著卵子使命的終結， 本月懷孕的時機就結束了。

Meanwhile, the now empty follicle begins to release progesterone, another hormone that tells the womb's lining to plump up with blood and nutrients in preparation for a fertilized egg that may embed there and grow.

同時，現在空的濾泡開始排放黃體酮，這是另一種賀爾蒙，來通知子宮內壁要填滿血液和營養素，為受精卵可在那著床和成長做準備。

If it doesn't embed, a few days later, the body's progesterone and estrogen levels plummet, meaning the womb stops padding out and starts to degenerate, eventually falling away.

如果沒有著床，幾天以後，身體的黃體酮和雌激素濃度急速下降，意味著子宮停止增厚並開始退化， 最終脫落。

Blood and tissue leave the body, forming the period.

血液和子宮內膜脫離身體，形成月經。

The womb can take up to a week to clear out its unused contents, after which, the cycle begins anew.

子宮會需要一周時間來清理沒用到的內膜，然後，循環又重新開始。

Soon afterwards, the ovaries begin to secrete estrogen again, and the womb lining thickens, getting ready to accommodate a fertilized egg or be shed.

很快之後，卵巢又開始分泌雌激素，然後子宮內膜就增厚，準備好讓受精卵著床或是流出。

Hormones continually control these activities by circulating in ideal amounts delivered at just the right time.

賀爾蒙透過在適當的時間以理想的量循環，來持續控制這些活動。

The cycle keeps on turning, transforming each day and each week into a milestone along its course towards pregnancy or a period.

循環又繼續進行下去，將每一天、每一周變成懷孕或月經期間的一個里程碑。

Although this cycle appears to move by clockwork, there's room for variation; women and their bodies are unique, after all.

雖然這個循環就像時鐘一樣的進行，還是有變化的可能性，畢竟，女性和她們的身體都是獨一無二的。

Menstrual cycles occur at different times in the month, ovulation comes at various points in the cycle, and some periods last longer than others.

月經循環在每個月的不同時間發生，在循環中任何時刻都有可能會排卵， 而且有些月經持續的時間比其他的還要長。

Menstruation even begins and ends at different times in life for different women, too; in other words, variations between periods are normal.

一生中，不同女性的月經在開始和結束的時間都不同，換句話說，每個月經的變異性是很正常的。

Appreciating these differences and learning about this monthly process can empower women, giving them the tools to understand and take charge of their own bodies.

欣賞這個異同，和學習這每月的過程可以賦予女性權力，賦予她們能夠了解自己身體和主宰自己身體的工具。

That way, they're able to factor this small cycle into a much larger cycle of life.

如此一來，她們就能夠將這個小周期化為一個更大的生命週期。