unintended pregnancies. But what happens when things don't go quite as planned?

意外懷孕。 但當事情不按計劃進行時，會發生什麼呢？

Thanks to modern science a second chance is possible. So how does emergency

多虧了現代科學，才有了第二次機會。 那麼，如何緊急

contraception or "plan-b" work? And what exactly is happening inside the female

避孕還是"計劃-b"工作？ 而女性體內到底發生了什麼

body? During the female menstrual cycle many hormones are released

體？在女性的月經週期中，會釋放出許多荷爾蒙

beginning a chain reaction. The pituitary gland in the brain releases FSH

開始了連鎖反應。 大腦中的腦垂體釋放FSH。

which stimulates the growth of follicles and a premature egg in the ovaries.

它能刺激卵泡的生長和卵巢中的一個早熟卵子。

These follicles then release estrogen

這些卵泡就會釋放雌激素

which helps prepare the uterus lining and triggers the brain to release leutinizing

幫助子宮內膜做好準備，並觸發大腦釋放利尿素。

hormone or LH. LH then triggers ovulation in which the egg is

激素或LH。 然後，LH觸發排卵，其中卵子是。

released into the fallopian tubes where it will spend around twelve to twenty-four

釋放到輸卵管中，它將在那裡度過大約12至24個月。

hours waiting to be fertilized. If the egg isn't fertilized in this time it

小時等待受精。 如果卵子在這段時間內沒有受精，它就會

begins to dissolve and is shed away with the uterine lining during menstruation.

開始溶解，並在月經期間隨子宮內膜脫落。

Though this window of opportunity is brief it's increased by the fact that sperm

雖然這個機會之窗是短暫的，但由於精子增加了

can live for up to five days.

可以活到五天。

So the keen sperm from days ago show up early to the show anxiously awaiting

於是，幾天前敏銳的精子們早早地出現在展會上焦急地等待著。

ovulation.

排卵。

Birth control pills taken on a regular basis modify these hormone levels. By

定期服用的避孕藥會改變這些激素水平。 通過

increasing progestin levels the body thinks it's already released an egg or

增加孕激素水準的身體認為它'已經釋放了一個卵子或。

already pregnant.

已經懷孕了。

It effectively decreases FSH and LH and the cycle halts. But in the case

它有效地降低了FSH和LH，週期停止。 但在這種情況下

of emergency contraception which is only taken if regular contraception methods

緊急避孕，只有在採用常規避孕方法的情況下才採取緊急避孕措施；

fail,

失敗。

a few different mechanisms take place. The most important thing to understand

幾種不同的機制發生。 最重要的是要了解

is that fertilization doesn't happen immediately or even within hours after

是受精不立即發生或甚至在幾個小時內，後

intercourse. Instead the sperm must undergo a few biological changes in the

性交。 而精子則必須經過一些生理變化，在。

fallopian tube first which takes time. This is why emergency contraception is

先輸卵管，這需要時間。 這就是為什麼緊急避孕是

still possible the morning after.

後天早上還有可能。

It can actually work up to five days after intercourse. If taken during the

其實它的作用時間可以達到性交後五天。 如果在

first half of the menstrual cycle, emergency contraception works

月經週期前半段，緊急避孕的作用

specifically to prevent ovulation.

專門用來防止排卵。

This way the egg is not released even though it may be ready and the available

這樣一來，即使雞蛋可能已經準備好了，可用的雞蛋也不會被釋放。

sperm cannot fertilize it.

精子不能使其受精。

In cases where it's too late to inhibit ovulation

在抑制排卵為時已晚的情況下'。

emergency contraception is thought to also thicken cervical mucus trapping sperm.

緊急避孕被認為也會使宮頸粘液增厚困住精子。

As well, it may directly inhibit fertilization between sperm and egg.

以及，可能直接抑制精子和卵子之間的受精。

Clinical studies show that emergency contraception prevents pregnancy

臨床研究表明，緊急避孕可以預防懷孕

before fertilization. No studies have shown an effect from emergency

受精前。 沒有研究表明，緊急

contraception if fertilization has already occurred. It does not stop

如果已經發生了受精，則應採取避孕措施。 它不會停止

implantation from happening nor does it have any effect after implantation has

植入後也不會有任何影響。

occurred.

發生。

In this way, its unable to cause an abortion.

這樣一來，其無法引起流產。

And while there are some minor side effects to these drugs, such as nausea and headaches,

而這些藥物雖然有一些輕微的副作用，如噁心、頭痛等。

scientific studies have demonstrated its safety in women of all ages and found

科學研究表明，它對所有年齡段的婦女都是安全的，並發現：

that the benefits far outweigh any risks. Got of burning question you want answered?

效益遠遠超過任何風險。 你有什麼迫切的問題想知道嗎？

Ask it in the comments or on facebook and twitter, and subscribe for more weekly science videos.

在評論中或在facebook和twitter上提問，並訂閱更多每週的科學視頻。