📚 Birth Control | 避孕
Birth control, also called contraception or family planning, refers to methods used to prevent pregnancy. In A-Level Biology, it is important to understand how each method disrupts a specific stage of reproduction, from gamete production and ovulation to fertilisation and implantation. A well-chosen method balances effectiveness, safety, convenience and personal values.
避孕（又称节育或计划生育）是指用于防止怀孕的方法。在 A-Level 生物学中，重点在于理解每种方法如何干扰生殖过程的某个特定阶段，包括配子产生、排卵、受精和着床。理想的方法要在有效性、安全性、便利性和个人价值观之间取得平衡。
1. What Is Birth Control? | 什么是避孕？
Conception requires a chain of events: the ovary releases a secondary oocyte at ovulation; sperm must be present in the fallopian tube; fertilisation must occur; and the early embryo must implant in the endometrium. Any method that interrupts one of these events can act as a contraceptive.
受孕需要一系列事件：卵巢在排卵时释放次级卵母细胞；精子必须到达输卵管；受精必须发生；早期胚胎必须植入子宫内膜。任何能中断这些事件之一的方法都可以起到避孕作用。
Contraceptive methods can be classified as hormonal, barrier, intrauterine, surgical, natural or emergency. Their success depends both on the biological mechanism and on how consistently the method is used.
避孕方法可分为激素法、屏障法、宫内法、手术法、自然法和紧急避孕法。其成功率既取决于生物学机制，也取决于使用者是否坚持正确使用。
2. Hormonal Control of the Female Cycle | 女性周期的激素调控
The anterior pituitary gland secretes follicle-stimulating hormone (FSH) and luteinising hormone (LH). FSH stimulates growth of ovarian follicles and secretion of oestrogen. A surge in LH triggers ovulation around day 14 of a typical 28-day cycle.
腺垂体分泌促卵泡激素（FSH）和黄体生成素（LH）。FSH 刺激卵泡生长并促进雌激素分泌。在典型的 28 天周期中，LH 峰大约在第 14 天引发排卵。
After ovulation, the ruptured follicle becomes the corpus luteum, which secretes progesterone. Progesterone maintains the thickened endometrium and, together with oestrogen, exerts negative feedback on the hypothalamus and pituitary, suppressing FSH and LH release.
排卵后，破裂的卵泡形成黄体，分泌孕酮。孕酮维持增厚的子宫内膜，并与雌激素一起对下丘脑和垂体产生负反馈，抑制 FSH 和 LH 的分泌。
Hormonal contraceptives exploit this negative feedback. By supplying a steady level of synthetic oestrogen and/or progesterone, they keep FSH and LH low, so follicles do not mature and ovulation is usually prevented.
激素类避孕药正是利用这一负反馈机制。通过提供稳定水平的合成雌激素和/或孕激素，它们使 FSH 和 LH 保持在低水平，因此卵泡通常不能成熟，排卵被抑制。
3. Combined Oral Contraceptive Pill | 复方口服避孕药
The combined oral contraceptive pill contains synthetic oestrogen and progesterone. It is usually taken for 21 consecutive days followed by a 7-day pill-free interval, during which a withdrawal bleed occurs.
复方口服避孕药含有合成雌激素和孕激素。通常连续服用 21 天，随后停药 7 天，停药期间会发生撤退性出血。
The constant oestrogen and progesterone inhibit secretion of FSH and LH by negative feedback. Without an LH surge, ovulation does not occur. In addition, progesterone thickens cervical mucus, making it harder for sperm to enter the uterus, and thins the endometrium, making implantation less likely.
持续的雌激素和孕激素通过负反馈抑制 FSH 和 LH 的分泌。没有 LH 峰，就不会发生排卵。此外，孕激素使宫颈黏液变稠，阻止精子进入子宫，同时使子宫内膜变薄，降低着床的可能性。
Advantages include reliable contraception when used perfectly, more regular and lighter periods, and reduced risk of ovarian and endometrial cancer. Disadvantages include side effects such as nausea, breast tenderness, mood changes, and a small increased risk of blood clots, especially in smokers or older users.
优点包括：正确使用时避孕效果可靠，月经更规律、量更少，并降低卵巢癌和子宫内膜癌的风险。缺点包括恶心、乳房胀痛、情绪变化等副作用，以及血栓风险略微升高，尤其是吸烟者或年龄较大的使用者。
4. Progesterone-Only Methods | 仅含孕激素的避孕法
The progesterone-only pill (mini-pill), injectable contraceptives and subdermal implants release a progestogen without oestrogen. They are useful for women who cannot take oestrogen, for example those with high blood pressure, migraines with aura or a history of blood clots.
仅含孕激素的避孕药（迷你药丸）、避孕注射剂和皮下埋植剂只释放孕激素，不含雌激素。它们适用于不能使用雌激素的女性，例如有高血压、先兆偏头痛或血栓病史者。
These methods work mainly by thickening cervical mucus, which blocks sperm transport, and by thinning the endometrium. Injectables and implants often also suppress ovulation by inhibiting FSH and LH release. Implants and injections are long-acting reversible contraceptives, lasting from several months to several years.
这些方法的主要作用是使宫颈黏液变稠，阻止精子通过，并使子宫内膜变薄。注射剂和埋植剂通常还能通过抑制 FSH 和 LH 的释放来抑制排卵。埋植剂和注射剂属于长效可逆避孕方法，有效期从几个月到几年不等。
5. Emergency Contraception | 紧急避孕
Emergency contraception is used after unprotected intercourse or contraceptive failure. A high dose of levonorgestrel or ulipristal acetate can delay or prevent ovulation if taken early. The copper IUD can also be inserted within a few days and is highly effective as emergency contraception.
紧急避孕用于无保护性行为或避孕失败之后。尽早服用高剂量左炔诺孕酮或醋酸乌利司他可以推迟或阻止排卵。铜质宫内节育器也可在数天内放置，作为紧急避孕非常有效。
Emergency hormonal contraception is not an abortifacient; if implantation has already occurred, it does not terminate the pregnancy. It is less reliable than regular contraception and may cause nausea and irregular bleeding.
紧急激素避孕药不是堕胎药；如果胚胎已经着床，它不能终止妊娠。它不如常规避孕可靠，并可能引起恶心和不规则出血。
6. Intrauterine Devices | 宫内节育器
An intrauterine device (IUD) is a small T-shaped device placed in the uterus. The copper IUD releases copper ions that are toxic to sperm and reduce their motility, preventing fertilisation. It may also alter the endometrium, making implantation less likely.
宫内节育器（IUD）是一种放置在子宫内的小型 T 形装置。铜质 IUD 释放铜离子，对精子有毒性，降低精子活力，从而阻止受精。它也可能改变子宫内膜，使着床不易发生。
The intrauterine system (IUS) is a hormonal IUD that releases a progestogen. It thickens cervical mucus, thins the endometrium and may suppress ovulation in some users. Both devices are highly effective, long-term and reversible, but they do not protect against sexually transmitted infections.
宫内缓释系统（IUS）是一种释放孕激素的激素宫内节育器。它使宫颈黏液变稠、子宫内膜变薄，并可能抑制部分使用者的排卵。这两种装置都非常有效、长效且可逆，但不能预防性传播感染。
7. Barrier Methods | 屏障避孕法
Barrier methods physically prevent sperm from reaching the oocyte. Male and female condoms, diaphragms and cervical caps are common examples. They can be used only when needed and have few systemic side effects.
屏障方法通过物理方式阻止精子接触卵母细胞。常见例子包括男用和女用避孕套、子宫隔膜和宫颈帽。它们只在需要时使用，几乎没有全身性副作用。
Male latex condoms are the only widely available method that also gives significant protection against sexually transmitted infections, including HIV. Spermicides can be used alongside barriers, but they are not reliable enough on their own.
男用乳胶避孕套是唯一能同时显著预防包括 HIV 在内的性传播感染的广泛可得的避孕方法。杀精剂可与屏障法配合使用，但单独使用
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