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    How does the contraceptive pill work?

    On this page
    1. Types of contraception
    2. The menstrual cycle
    3. How does the combined pill work?
    4. How does the mini pill work?
    5. How long does the contraceptive pill take to work?
    6. How does the emergency contraceptive pill work?
    7. More contraception information

    Written by Dr Mitra Dutt

    If you’re thinking about taking the contraceptive pill you might be wondering how it works to stop you getting pregnant. As well as what options are available, and which one will suit you. This article looks at how the pill changes your menstrual cycle to prevent pregnancy.  

    Types of contraception

    There are four main types of contraception:

    • Hormonal - 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.

    The only form of contraception that is 100% effective is abstinence (not having sex), 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. They are used by around 42% of women in the UK.

    The two types of contraception 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, contains synthetic versions of progesterone.

    It’s through the management of oestrogen and progesterone that pregnancy is prevented.

    The menstrual cycle

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

    The average menstrual cycle is around 28 days, but anything between 24 and 35 days is also normal. 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 period

    This is a simplified progression of a 28-day menstrual cycle.

    • Day 1: Your period begins and the lining of your uterus (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. 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 1.


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

    What is breakthrough bleeding?

    Breakthrough bleeding is any bleeding or spotting you may experience between your periods. Hormonal contraceptives can cause bleeding between periods. Spotting can be a side effect of the combined or progesterone only pill. 

    You may also bleed between your periods if you miss a pill or take emergency contraception. If you’re worried about breakthrough bleeding or have experienced it for months speak to your GP.  

    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, such as Gedarel 20.

    How does the mini pill work?

    The mini pill (progestogen only pill or POP) 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.  

    The mini pill can cause side effects, such as irregular bleeding, headaches, acne, nausea and mood swings.

    How long does the contraceptive pill take to work?

    How long the contraceptive pill takes to work depends on when you first start taking the pill. Which day during your period you take the pill affects how effective it is at preventing pregnancy.  

    How long does the combined pill take to work?

    If you start taking the combined pill on: 

    • The first day of your period – protected straight away
    • The fifth day of your cycle – protected straight away 
    • After the fifth day of your cycle – Not protected straight away, you need to use additional contraception for 7 days 

    How long does the mini pill take to work?

    The progestogen only pill (mini pill) starts to work straight the way if you start it on days 1 to 5 of your period. If you have a short menstrual cycle (less than 28 days) you’ll need to use condoms until you’ve taken the pill for 2 days. 

    If you start taking the pill on any other day of your cycle, you’ll need to use additional contraception for 2 days. 

    How does the emergency contraceptive pill work?

    The morning after pill works by stopping or delaying your body releasing an egg (ovulation). To be effective they need to be taken at the right time find out more in our guide. These pills aren’t meant to be used as regular contraception, but you can use them more than once during your menstrual cycle if you need to.  

    More contraception information

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


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