Contraception: GCSE Biology Guide to Methods and Mechanisms

How does reproduction normally proceed?

Pregnancy requires three events in sequence:

1. Ovulation: the ovary releases a mature egg into the fallopian tube (approximately once every 28 days, regulated by FSH and LH).
2. Fertilisation: a sperm cell fuses with the egg, usually in the fallopian tube, forming a zygote.
3. Implantation: the zygote divides by mitosis to form a ball of cells (blastocyst) that travels to the uterus and embeds in the thickened endometrium (uterine lining).

Contraception prevents pregnancy by blocking one or more of these steps.

What are hormonal methods of contraception?

Hormonal contraceptives use synthetic versions of oestrogen and/or progesterone to alter the body's hormonal environment, preventing pregnancy in one or more ways:

• Inhibiting ovulation: high levels of oestrogen/progesterone suppress the release of FSH and LH from the pituitary gland, preventing ovulation. No egg = no fertilisation.
• Thickening cervical mucus: progesterone makes the mucus at the cervix too thick for sperm to penetrate.
• Thinning the endometrium: progesterone prevents the uterine lining from thickening sufficiently for implantation.

Hormonal methods are highly effective (>99% with perfect use) but do not protect against sexually transmitted infections (STIs).

What are barrier methods?

Barrier methods physically prevent sperm from reaching the egg. They do not affect hormones and have no systemic effects on the body.

• Male condom: a thin sheath worn over the penis, preventing sperm entering the vagina. The only contraceptive method that also provides protection against STIs.
• Female condom: inserted into the vagina, lining it and covering the cervix.
• Diaphragm/cervical cap: a dome-shaped device inserted before sex to cover the cervix; typically used with a spermicide gel.

Barrier methods are effective (85–98% with typical use) but less reliable than hormonal methods unless used correctly and consistently.

What is an IUD and how does it work?

A copper intrauterine device (IUD) is a small T-shaped device fitted into the uterus by a doctor or nurse. It is not hormonal. Copper ions released by the device are toxic to sperm — they reduce sperm motility and viability, preventing fertilisation. The copper also makes the uterine lining less favourable for implantation if fertilisation does occur.

Copper IUDs are over 99% effective and can remain in place for up to 10 years. They are also used as emergency contraception if fitted within 5 days of unprotected sex.

How do methods compare?

What is emergency contraception?

Emergency contraception reduces the chance of pregnancy after unprotected sex. Options include:

• Emergency hormonal pill ("morning-after pill"): taken within 72 hours (most effective) or up to 120 hours. Works primarily by delaying or preventing ovulation.
• Copper IUD: inserted within 5 days; over 99% effective as emergency contraception.

Emergency contraception is not intended as a regular method — it is a backup for unplanned situations.

Frequently asked questions

Why do some people prefer non-hormonal contraception?

Hormonal contraceptives alter the body's natural hormone levels. Some people experience side effects including mood changes, nausea, headaches or changes in menstrual patterns, though these vary greatly between individuals and methods. People with certain health conditions (e.g. a history of blood clots, some cancers) may be advised against oestrogen-containing methods. Non-hormonal options — barrier methods, copper IUDs — suit people who want to avoid synthetic hormones entirely.

How do hormonal contraceptives affect the menstrual cycle?

Hormonal methods suppress the natural fluctuations in FSH and LH that drive the menstrual cycle. On the combined pill, the week off tablets allows a light withdrawal bleed (not a true menstrual period, since ovulation has not occurred). On the progestogen-only pill or injection, periods may become irregular or stop altogether — this is not harmful and normal periods resume when the method is discontinued.

What is the difference between contraception and the termination of pregnancy?

Contraception prevents fertilisation or implantation — it acts before a pregnancy is established. The termination of pregnancy (abortion) ends a pregnancy that has already begun, typically after implantation. Emergency contraception is contraceptive (it prevents a pregnancy from starting) rather than abortifacient; the copper IUD, if fitted very early after fertilisation, might also prevent implantation.

Why is abstinence described as 100% effective but with limitations?

Abstinence — not having penetrative sex — is the only method that is fully reliable at preventing pregnancy and STI transmission, since both require sexual contact. Its limitation is that it requires consistent decision-making; unplanned sexual activity may occur, and it provides no protection in the event of sexual assault. For these reasons, health education includes abstinence alongside other methods rather than instead of them.

---

For biology tutoring that builds understanding from cells to reproduction and human health, explore Professor Darwin at aitutors.me.