Here's what has to happen for pregnancy to occur after sexual intercourse. Sperm must swim up the vagina, through the cervical opening, upwards through the uterus, and into one of the two fallopian tubes. If an egg, released during that month's ovulation, is in the tube, one sperm has a chance to fertilize it. Contraceptives are designed to prevent this process, and they work in three basic ways. They block the sperm, disable sperm before they reach the uterus, or suppress ovulation. Block is the simplest. Male and female condoms prevent sperm from coming into contact with the vaginal space. That barrier is also why they, unlike other contraceptive methods, are able to prevent transmission of certain sexually transmitted diseases. Meanwhile, the diaphragm, cervical cap, and sponge work by being placed over the cervix, barricading the entrance to the uterus. These contraceptives are sometimes called barrier methods and can be used with spermicides, an example of the second category, disable. A spermicide is a chemical that immobilizes and destroys sperm. Today's spermicides come as foam, cream, jelly, suppositories, and even a thin piece of translucent film that dissolves in the vagina. These products can be inserted directly into the vagina before intercourse, or can be combined with block methods, like a diaphragm or condom, for added proection. The third category for preventing pregnancy works by suppressing the action of an egg maturing in the ovary. If there isn't an egg available in the fallopian tube, there's nothing for sperm to fertilize. Hormonal contraceptives, including the pill, the patch, the Depo shot, and the vaginal ring all release synthetic versions of various combinations of progesterone and estrogen. This hormone cocktail suppresses ovulation, keeping the immature egg safely sequestered in the ovary. Synthetic progesterone also has a block trick up its sleeve. It makes cervical mucus too thick and sticky for sperm to swim through easily. There are other contraceptives that use multiple approaches at the same time. For example, many IUDs, or intrauterine devices, contain synthetic hormones which suppress ovulation. Some also contain copper, which disable sperm while also making egg implantation in the uterus difficult. Block, disable, or suppress: is one strategy better than the other? There are differences, but a lot of it has to do with how convenient and easy it is to use each contraceptive correctly. For example, male condoms would be about 98% effective if everyone used them perfectly. That 98% means if 100 couples correctly used condoms for a year, two women would get pregnant. But not everyone uses them correctly, so they're only 82% effective in practice. Other methods, like the patch and pill, are 99% effective when they're used perfectly. But in practice, that's 91%. Spermicide is only 85% effective, even with perfect usage, and just 71% effective with typical usage. Another important consideration in the choice of contraceptives are side effects, which almost exclusively affect women rather than men. Hormonal methods in particular can cause symptoms like headaches, nausea, and high blood pressure, but they vary from woman to woman. That's why these methods require a prescription from a doctor. The choice of contraceptive method is a personal one, and what works best for you now may change later. Scientists also continue to research new methods, such as a male pill that would prevent sperm production. In the meantime, there are quite a few options to block sperm, disable them, or suppress eggs and keep them out of reach.
今天要讲的是在性交之后怀孕是如何发生的 精子必须游向阴道 通过宫颈口 向上通过子宫 然后进入两个输卵管中的一个 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 避孕用品就是用来阻止这个过程的 它有三种基本的工作原理 它们阻止精子 使精子在到达输卵管前失活 或者抑制排卵 阻止是最简单的方式 男女用避孕套阻止精子 与阴道接触 与其它避孕方法不同的是 这种方式 能避免某些性病的传播。 同时 隔膜 子宫帽和海绵状的东西 放在子宫颈上一起发挥作用 阻碍精子进入子宫 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 杀精子剂是第二类使精子丧失活性的方法 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 现在的杀精子剂主要是泡沫 油脂 胶体 栓剂 甚至是一种能在阴道内溶解的半透明薄膜 这些产品可以在性交前直接插入阴道 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 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 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 激素类避孕药物 包括口服药片 膏药 避孕针和阴道环 都能释放人造的各种孕酮和雌性激素的结合物 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 合成孕酮对付避孕也自有一招 它将子宫液变得又粘又稠 使得精子不易通过 其它的避孕方式就是将上面几种方式同时使用 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 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 阻止 失去活性 或者抑制 难道这个办法就比另外一个好吗 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 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 98%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 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 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 可是在实践中 只有91%有效果 而杀精子剂即使正确使用 效果也仅有85% 如果只是没有方法地使用 那么效果仅为71% 另一个选择避孕方式的重要因素是其副作用 这几乎仅仅影响女人 而不是男人 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 这就是为什么这些方法需要医生开处方的原因 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 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 同时 阻止精子 也有相当多的方法进行选择 使它失去活性 或者抑制卵子 使它和精子无法接触