Here's what has to happen for pregnancy to occur after sexual intercourse.
今天要讲的是在性交之后怀孕是如何发生的。
Sperm must swim up the vagina, through the cervical opening,
精子必须游向阴道，通过宫颈口，
upwards through the uterus, and into one of the two fallopian tubes.
向上通过子宫，然后进入两个输卵管中的一个。
If an egg, released during that month's ovulation, is in the tube, one sperm has a chance to fertilize it.
如果在那个月产生的卵子正好在输卵管中，那么就会有一个精子有机会与卵子结合。
Contraceptives are designed to prevent this process, and they work in three basic ways.
避孕用品就是用来阻止这个过程的，有三种基本的工作原理。
They block the sperm, disable sperm before they reach the uterus, or suppress ovulation.
它们阻止精子，使精子在到达输卵管前失活，或者抑制排卵。
Block is the simplest. Male and female condoms prevent sperm from coming into contact with the vaginal space.
阻止是最简单的方式。男女用避孕套阻止精子与阴道接触。
That barrier is also why they, unlike other contraceptive methods,
这种方式与其它避孕方法不同的是，
are able to prevent transmission of certain sexually transmitted diseases.
它能避免某些性病的传播。
Meanwhile, the diaphragm, cervical cap, and sponge work
同时，隔膜、子宫帽和海绵状的东西
by being placed over the cervix, barricading the entrance to the uterus.
放在子宫颈上一起发挥作用，阻碍精子进入子宫。
These contraceptives are sometimes called barrier methods and can be used with spermicides,
这些避孕方法有时被称为障碍避孕法，并且也能与杀精子剂一起使用，
an example of the second category, disable.
杀精子剂是第二类使精子丧失活性的方法。
A spermicide is a chemical that immobilizes and destroys sperm.
杀精子剂是一种使精子丧失机动性进而破坏精子的化学药物。
Today's spermicides come as foam, cream, jelly, suppositories,
现在的杀精子剂主要是泡沫、油脂、胶体、栓剂，
and even a thin piece of translucent film that dissolves in the vagina.
甚至是一种能在阴道内溶解的半透明薄膜。
These products can be inserted directly into the vagina before intercourse,
这些产品可以在性交前直接插入阴道，
or can be combined with block methods, like a diaphragm or condom, for added proection.
或者与阻止法相结合，比如隔膜或者避孕套，用以增加避孕效果。
The third category for preventing pregnancy works by suppressing the action of an egg maturing in the ovary.
第三类防止怀孕的方法是通过抑制卵子在卵巢中的成熟进程。
If there isn't an egg available in the fallopian tube, there's nothing for sperm to fertilize.
如果在输卵管中没有可利用的卵子，也就没有精子和卵子的结合。
Hormonal contraceptives, including the pill, the patch, the Depo shot, and the vaginal ring
激素类避孕药物，包括口服药片、膏药、避孕针和阴道环，
all release synthetic versions of various combinations of progesterone and estrogen.
都能释放人造的各种孕酮和雌性激素的结合物。

This hormone cocktail suppresses ovulation, keeping the immature egg safely sequestered in the ovary.
这种激素类混合物抑制排卵，使卵巢中未成熟的卵子被安全隔绝。
Synthetic progesterone also has a block trick up its sleeve.
合成孕酮对付避孕也自有一招。
It makes cervical mucus too thick and sticky for sperm to swim through easily.
它将子宫液变得又粘又稠，使得精子不易通过。
There are other contraceptives that use multiple approaches at the same time.
其它的避孕方式就是将上面几种方式同时使用。
For example, many IUDs, or intrauterine devices, contain synthetic hormones which suppress ovulation.
例如，许多子宫内避孕器或者宫内避孕环含有抑制排卵的合成激素。
Some also contain copper, which disable sperm while also making egg implantation in the uterus difficult.
一些含有铜的合成激素可使精子丧失活性，同时使卵子在子宫内难以生存。
Block, disable, or suppress: is one strategy better than the other?
阻止、失去活性或者抑制，难道这个办法就比另外一个好吗？
There are differences, but a lot of it has to do with how convenient and easy it is to use each contraceptive correctly.
有一定的区别，但这些方法必须要与怎样才能方便、简单地正确使用这些避孕用具有关。
For example, male condoms would be about 98% effective if everyone used them perfectly.
例如，男用避孕套如果能被正确地使用的话，有百分之九十八都会有效果。
That 98% means if 100 couples correctly used condoms for a year, two women would get pregnant.
98%意味着，如果100对夫妇在一年中正确的使用避孕套，只有两个女性会怀孕。
But not everyone uses them correctly, so they're only 82% effective in practice.
但并不是每个人都能正确使用避孕套，所以事实上只有82%有效果。
Other methods, like the patch and pill, are 99% effective when they're used perfectly.
其它方法，比如膏药、药片，如果它们被正确地使用，99%都有效果。
But in practice, that's 91%.
可是在实践中，只有91%有效果。
Spermicide is only 85% effective, even with perfect usage, and just 71% effective with typical usage.
而杀精子剂即使正确使用，效果也仅有85%，如果只是典型使用，那么效果仅为71%。
Another important consideration in the choice of contraceptives are side effects,
另一个选择避孕方式的重要因素是其副作用，
which almost exclusively affect women rather than men.
这几乎仅仅影响女人，而不是男人。
Hormonal methods in particular can cause symptoms like headaches, nausea, and high blood pressure, but they vary from woman to woman.
尤其是激素类方法会引起一些症状，比如头疼、恶心以及高血压，但这也只是因人而异。
That's why these methods require a prescription from a doctor.
这就是为什么这些方法需要医生开处方的原因。
The choice of contraceptive method is a personal one, and what works best for you now may change later.
选择避孕的方法是比较私人的，并且好的方法对同一个人的效果也可能前后不一。
Scientists also continue to research new methods, such as a male pill that would prevent sperm production.
科学家也在继续研究新的方法，比如阻止精子产生的男性避孕药。
In the meantime, there are quite a few options to block sperm, disable them, or suppress eggs and keep them out of reach.
同时，也有相当多的选择来阻止精子、使它失去活性或者抑制卵子使它和精子无法接触。