How hormonal contraception works

1. Hormonal contraception uses oestrogen, progesterone or both to interrupt the menstrual cycle.
2. Oestrogen taken continuously keeps its concentration in the blood permanently high.
3. A permanently high oestrogen concentration inhibits the release of FSH from the pituitary gland.
4. Without FSH no follicle is stimulated, so no egg matures and ovulation cannot happen.
5. Progesterone thickens the mucus produced at the cervix, forming a plug that sperm cannot swim through.
6. Progesterone also inhibits the release of FSH and LH, which is a second route to preventing ovulation.
7. Some methods also make the uterus lining too thin for a fertilised egg to implant.

Hormonal methods available

1. The combined pill contains oestrogen and progesterone and is taken daily.
2. Used exactly as directed it is over $99\%$ effective, but with typical use effectiveness falls to about $91\%$.
3. The progesterone only pill avoids oestrogen and so suits people at risk of blood clots, but it must be taken at the same time each day.
4. A skin patch releases oestrogen and progesterone through the skin and is replaced once a week.
5. An implant placed under the skin of the upper arm releases progesterone slowly for up to $3$ years.
6. An injection of progesterone lasts about $3$ months before it must be repeated.
7. An intrauterine system placed inside the uterus releases progesterone and lasts between $3$ and $5$ years.
8. Side effects can include headaches, changes in mood and a small rise in the risk of blood clots.
9. No hormonal method gives any protection against sexually transmitted infections.

Non-hormonal methods

1. A condom is a barrier method worn over the penis that traps sperm so they never enter the vagina.
2. Condoms are about $98\%$ effective when used correctly, though typical use gives closer to $82\%$.
3. A condom is the only common method that also reduces the spread of sexually transmitted infections.
4. A diaphragm is a barrier fitted over the cervix and used with spermicide, giving roughly $92\%$ effectiveness.
5. Spermicide is a chemical that kills or immobilises sperm, and used on its own it is only about $71\%$ to $82\%$ effective.
6. A copper intrauterine device releases copper ions that are toxic to sperm and also prevent implantation, giving over $99\%$ effectiveness for up to $10$ years.
7. Sterilisation means cutting or tying the oviducts in females or the sperm ducts in males, and it is over $99\%$ effective.
8. Sterilisation must be treated as permanent, because reversing the operation is difficult and often fails.
9. Natural methods avoid intercourse in the days around ovulation, which needs careful record keeping and gives between $76\%$ and $99\%$ effectiveness.
10. Abstinence from intercourse is the only method that prevents pregnancy completely.

Comparing the methods

1. Judge a method on its effectiveness, whether it is reversible, whether it protects against infection, its side effects, its cost and how easy it is to use.
2. The implant, injection, intrauterine device and intrauterine system are the most effective in practice, because they do not depend on the user remembering anything.
3. Pills are highly effective in theory but less so in practice, since a single missed pill can allow a follicle to mature.
4. Only condoms reduce the transmission of sexually transmitted infections, so they are often used alongside a hormonal method.
5. Barrier methods have no hormonal side effects, which suits people who cannot take oestrogen.
6. Sterilisation is the most permanent option and is unsuitable for anyone who may want children later.
7. Some people avoid hormonal and surgical methods for religious or ethical reasons and choose natural methods instead.
8. Cost differs too, since condoms are bought repeatedly while an implant is fitted once and lasts for years.