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How does the contraceptive pill work?

How does the contraceptive pill work?

Types of contraception

There are four main types of contraception:

  • Hormonal - as above, this affects your hormones to prevent ovulation and pregnancy. Includes the contraceptive pill, patch and ring as well as the longer acting reversible contraceptives, comprising of the implant, intrauterine system and injection
  • Barrier - this stops sperm from reaching eggs, preventing fertilisation. Includes condoms, diaphragm and femidom.
  • Intrauterine - a device inserted into your uterus creating an environment that stops an egg from embedding.
  • Sterilisation - this is an irreversible surgical procedure that seals or blocks your fallopian tubes, preventing an egg or sperm from meeting.

No forms of contraception are 100% effective and only condoms will protect you from sexually transmitted infections.

The contraceptive pill

The pill is a form of hormonal contraception that works by changing the menstrual cycle. Used correctly it prevents pregnancy in approximately 99% of women when used perfectly. 

The two types of contraceptive pill are:

  • The combined pill contains synthetic versions of the hormones oestrogen and progesterone. These hormones are produced during the menstrual cycle.
  • The progestogen-only pill, known as the mini pill or POP, which contains synthetic versions of progesterone.

The menstrual cycle

The average menstrual cycle is around 28 days, but for some women it can range from 21 to 40 days. 

The first day of your period is the first day of your cycle. This is when the lining of the womb is shed, causing a bleed. It can last between three and eight days but is usually about five with the heaviest bleeding on the first two days.

Changes in your body’s hormone levels before the monthly period, known as premenstrual syndrome (PMS), can result in mood swings, feeling bloated and a loss of interest in sex.

Your periods will stop when you reach menopause, usually in your late 40s to mid-50s where they will decline before stopping altogether.

Simplified 28-day period

Day 1
Your period begins and the lining of your uterus is shed, causing a bleed. Your body releases Follicle Stimulating Hormone (FSH), causing your ovaries to start to ripen the egg and produce oestrogen. The oestrogen tells your body to stop producing FSH and to start producing Luteinising Hormone (LH).

Days 4-7
The lining of your womb (uterus) begins to thicken in preparation to support a pregnancy. This happens immediately after your period has finished.

Days 7-14
Your oestrogen levels rise and peak. This causes ovulation, when a ripe egg is released from your ovaries. If the egg is fertilised, it can embed itself in the now thick uterus wall. The mucus in the cervix thins, allowing sperm into the womb and fallopian tubes.

Day 14
The ovaries begin to produce progesterone, keeping the lining of the uterus thick and spongy, to support a fertilised egg. It also causes the mucus at the cervix to thicken, preventing sperm from getting through. If you become pregnant, the level of progesterone would remain high.

Day 20
Your progesterone levels peak.

Day 28
If there’s no fertilised egg the levels of progesterone will then begin to drop, eventually causing your period. Your cycle starts over again from day one.

How does the combined pill work?

It prevents pregnancy in three major ways:

  1. The synthetic oestrogen stops your body from producing two hormones that are involved in the menstrual cycle. This prevents ovulation (the release of an egg) each month.
  2. The synthetic progesterone thickens the mucus at the entrance of your womb so that sperm can’t get through to fertilise your eggs.
  3. The synthetic progesterone also thins the lining of the uterus, making it difficult for a fertilised egg to implant itself.

There are two forms of the combined pill: monophasic and phasic.

Monophasic pills deliver the same dose of synthetic oestrogen and progesterone every day.

Phasic pills try to keep hormone levels consistent by altering dosage throughout the menstrual cycle.

With both types, there is a seven-day break every 21 days to allow for a period. Every day pills are also available which have 21 active and then seven inactive (dummy) pills per pack.

The most common brands of the combined contraceptive pill are:

The combined pill can cause side effects such as nausea, headaches and sore breasts. These symptoms can be minimised by taking a low-dose pill. 

How does the mini pill work?

The mini pill contains just progesterone, which has two functions:

  1. It thickens the cervical mucus, making it difficult for sperm to get through to fertilise an egg.
  2. It also thins the lining of the uterus, making it less likely that a fertilised egg will implant itself.

Cerazette is the most commonly used mini pill. It also works by preventing ovulation.

For a more in-depth understanding of how replicating oestrogen and progesterone prevents pregnancy, it’s helpful to understand how menstrual cycles (periods) work.

More information

Find out more by visiting our online contraception clinic. For more specific guidance for your particular needs, it’s best to have a consultation with your GP.

Looking for contraception?

Go to our contraception service



Authors and editors

Reviewed and updated by: Our clinical team Date reviewed: 22-09-2023