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.
性交之後必須有下列情形才會懷孕 精子必須沿著陰道上行 通過子宮頸口 向上通過子宮 進入兩個輸卵管之一 如果該月釋出的卵子正好在輸卵管中 某一精子就有機會與卵子結合 避孕用品是設計來不讓這過程發生 依三種基本原理來作用 1. 阻隔精子 2. 使精子在到達輸卵管前喪失能力 3. 抑制排卵 阻隔是最簡單的方式 男用和女用保險套阻止精子接觸陰道 與其它避孕方法不同的是 阻隔法也能避免傳播某些性病 避孕隔膜、子宮帽和避孕海綿 放在子宮頸上以阻礙精子進入子宮 這類避孕法有時被稱為 「障礙避孕法」 能與殺精劑並用 殺精劑是第二類 (使精子喪失能力)的一個實例 殺精劑這種化學藥物 可使精子失去游動力和殺滅精子 當前殺精劑的主要形式是 泡沫、乳液、凝膠、栓劑 甚至是會溶解於陰道內的半透明薄膜 這些產品能在性交前直接被置入陰道 或與阻隔法合用 例如避孕隔膜或保險套 以增強避孕的效果 第三類避孕法的原理是 抑制卵子在卵巢中的成熟進程 如果輸卵管內没有成熟的卵子 就沒有卵子可受精 激素類避孕劑 包括口服藥丸、避孕貼片、 避孕針和陰道環 都會釋放各種合成的 黃體素和雌性素的混合物 這種激素類混合物會抑制排卵 安全地隔絕卵巢中未成熟的卵子 合成黃體素也具有阻隔的招數 它使子宮頸黏液變得粘稠 精子不易通過 也有其他的方式混用數種避孕的方法 例如,許多子宮內避孕器 含有抑制排卵的合成激素 有些還含有銅 使精子喪失活力 並使卵子不易在子宮內著床 阻隔、使失去活力或者抑制 其中的一種方法勝過其他方法嗎? 它們各有擅長,但主要取決於便利性 和能夠正確使用的容易程度 例如,男用保險套能達 98% 的效果 如果每個人都正確使用的話 那 98% 的意思是在一年的期間裡 一百對伴侶正確地使用保險套 會有兩個女人懷孕 但是並非每個人都正確地使用 所以實際上的效果只有 82% 其它方法如避孕貼片、口服避孕藥 有 99% 的效果 如果被正確地使用的話 但是實際上只有 91% 而正確使用殺精劑的效果僅有 85% 一般的使用情形下只剩 71% 另一選擇避孕法的重要因素 是考量其副作用 受影響的幾乎全是女人,不是男人 尤其激素類方法會引起一些症狀 例如頭疼、噁心以及高血壓 因人而異 這也是為何激素法需要醫生的處方 避孕法取決於個人的選擇 現在適用你的方法可能以後不合用 科學家也持續地研究新方法 例如阻止產生精子的男性避孕藥丸 於此其間,我們有數種選項: 阻隔精子 使精子失去活力 或是抑制卵子,使精子無法接觸