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How does the pill work?

Routine Contraception Pill Packet

The oral contraceptive pill, introduced in the UK in 1961, has been hailed as being one of the major medical advances of the 20th century. It prevents pregnancy in approximately 99% of women (as long as it’s used correctly). For many people however, the question remains, how does the pill work?

There are four types of contraception: hormonal, barrier, intrauterine (inserted inside the uterus), and sterilisation. The pill is a form of hormonal contraception, and, like all hormonal contraception, works by changing the menstrual cycle. How the contraceptive pill does this depends on the type of pill you’re using.

There are two kinds of contraceptive pills: the combined pill and the progesterone-only pill (or mini pill). They differ from one another in the hormones that they contain. The combined pill contains synthetic versions of both oestrogen and progesterone (two hormones produced during the menstrual cycle), while the mini pill contains only synthetic progesterone.

How does the combined pill work?

The combined pill prevents pregnancy in three major ways:

  1. The synthetic oestrogen stops your body from producing two hormones that are involved in the menstrual cycle: Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH). This prevents your ovaries from producing an egg because it stops your eggs from ripening and ovulating.

  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.

Within the combined pill there are two subcategories: monophasic and phasic. Monophasic combined contraception delivers the same dose of synthetic oestrogen and progesterone every day, whereas 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 menstruation to occur.

How does the mini pill work?

Unlike the combined pill, the mini pill only contains progesterone. The majority of mini pills work by:

  1. Thickening the cervical mucus, making it difficult for sperm to get through to fertilise an egg.

  2. Thinning the lining of the uterus, making it less likely that a fertilised egg will implant itself.

Cerazette, the most commonly used mini pill, also works by preventing ovulation. For a more in-depth understanding of how replicating oestrogen and progesterone prevents pregnancy, it’s helpful to have an understanding of how the menstrual cycle works.

How a typical menstrual cycle works

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.

Here is a simplified explanation of what happens during a normal menstrual cycle (based on 28 days):

  1. On the first day of your cycle (when you start your period) your pituitary gland starts producing FSH, causing your ovaries to begin to ripening an egg and to start producing oestrogen. Oestrogen tells your body to stop producing FSH and to start producing LH.

  2. After about 4-7 days the lining of your womb (uterus) begins to thicken in preparation for supporting a pregnancy. This happens immediately after your period has finished.

  3. Your oestrogen levels continue to rise and peak at around day 14 (half way through your cycle). When the level of oestrogen peaks, it causes the now fully ripened egg to be released from your ovaries into your fallopian tube (this is known as ovulation). If the egg is fertilised, it should embed itself in the now thick uterus wall. Just before ovulation oestrogen also causes the mucus in the cervix to become thin, allowing sperm into the womb and fallopian tubes.

  4. The ovaries begin to produce progesterone after ovulation (from around day 14). Progesterone keeps the lining of the uterus thick and spongy in case a fertilised egg implants itself in the uterus wall. It also causes the mucus at the cervix to thicken, preventing sperm from getting through. If you were pregnant the level of progesterone would remain high.

  5. The level of progesterone peaks at around day 20 of your cycle (or three quarters through your cycle). If there’s no fertilised egg the levels of progesterone will then begin drop, eventually causing the lining to shed (i.e. you get your period). This happens at around day 28. Your cycle starts over again from day 1.

To learn more, visit our online contraception clinic.