The hormonal contraceptive pill recently celebrated 61 years of use in the UK. Even after 60 years the pill is still the number one choice when it comes to hormonal contraception, with around 30% of people in the UK using the pill as their main form of contraception(1).

But how does the pill really work and what happens to your menstrual cycle when you’re on the pill? Let’s explore below…

What is your menstrual cycle?

The majority of people who are assigned female at birth and are of reproductive age (roughly 15 – 49 years) experience a series of natural hormonal changes within their bodies over a period of about a month. This is a cycle that prepares your body for pregnancy and includes ovulation (the release of an egg) and menstruation (having your period). This whole cycle is known as the menstrual cycle, which begins on the first day of your period. It is a common misconception that the menstrual cycle just involves the monthly bleeding that is your period, but it actually includes all the hormonal changes leading up to and after this.

There are two major hormonal players in your menstrual cycle; these are oestrogen and progesterone. Along with Luteinising Hormone (LH) and Follicle Stimulating Hormone (FSH), these control the major events in your menstrual cycle.

After your period has finished, levels of the hormone oestrogen rise within your body. This causes your uterus lining to thicken, and an egg to develop in your ovary. Eventually levels of oestrogen rise high enough, along with LH and FSH, so that an egg is released. At this point progesterone levels begin to rise to prepare your body for pregnancy by stabilising the uterine lining. If the egg is not fertilised by sperm then oestrogen and progesterone levels drop and the lining of your uterus will start to shed as you begin your period. So you can think of your period as more than just a monthly bleed but as the complete shedding of your uterine lining (including blood and other tissue)... that’s right, our bodies do incredible things!(2).

Then the cycle begins all over again!

The types of pill & how they affect your cycle

There are two types of hormonal contraceptive pill, the combined oral contraceptive pill (COCP) and the progesterone-only pill (POP). As these pills are taken orally, they lead to hormone release throughout your body's entire system via your bloodstream.  The combined pill contains the hormones oestrogen and progestins (a synthetic version of the natural hormone progesterone) in varying quantities, depending on the brand of pill. As it says on the tin the progesterone-only pill contains just the synthetic hormone progestin. Both types of pills are taken daily, with most brands of combined pill being taken for 21 days and with a 7 day break, during which you may have a withdrawal bleed. The POP is taken for 28 days continuously, without any breaks. It is important to note here that the 7 day break from the combined pill is not necessary to prevent pregnancy and the latest research has shown that there are no health benefits from taking this pill break. 

The withdrawal bleed you experience when taking the 7 day break from the pill is not a period. This bleed occurs as a result of the sudden drop in hormones when you stop taking the pill, rather than as the result of going through your natural menstrual cycle and subsequently not becoming pregnant in that cycle. 

The POP works to prevent pregnancy by thickening the mucus in your cervix. The cervix is located at the entrance to your uterus, at the top of your vagina. This stops sperm being able to enter your uterus and fertilise an egg. Some types of POP also can prevent an egg being released in the first place, by keeping levels of progesterone high an egg will not be able to mature in the ovary and therefore will not be released(3, 4).

The COCP works in three ways to prevent pregnancy. Continuously high levels of oestrogen and progesterone prevents the monthly development and release of an egg. The mucus around your cervix is also thickened by progesterone so that sperm cannot reach an egg, and unlike the POP, the COCP thins the lining of the womb, making it less hospitable to a fertilised egg(3, 5).

With both forms of the hormonal pill, your body is not going through its natural menstrual cycle and if taken correctly you will not be ovulating whilst taking them. If you do not ovulate you cannot get pregnant. Both forms of the hormonal pill may also be used for menstrual suppression (stopping you getting your period each month). The stopping of the natural menstrual cycle and period can provide relief to those suffering from menstrual-related conditions such as endometriosis and is a decision that can be made between you and your provider (6). 

With contraception in general, and specifically the pill, many people have concerns about the effect of the pill on future fertility. Recent research has found that taking hormonal contraceptive pills does not affect your long term fertility, regardless of how long you take them for (7). In some cases there may be a short delay in return to fertility but this is similar to other contraceptive methods, such as the hormonal IUS (8). 

Now we know that we’ve covered a lot here but it’s important to remember that there are still many unknowns in the field of contraception and women’s health. The ways that our team is trying to navigate these unknowns are through rigorous research in women’s health. Check out our latest contraception study currently being rolled out in the UK and stay up to date with our health blogs by joining the Dama Health community. 

References: 

1.     French RS, Gibson L, Geary R, Glasier A, Wellings K. Changes in the prevalence and profile of users of contraception in Britain 2000–2010: evidence from two National Surveys of Sexual Attitudes and Lifestyles. BMJ Sexual & Reproductive Health. 2020;46(3):200-9.
2.     NHS. Periods and fertility in the menstrual cycle NHS Website2019 [updated 05/08/2019. Available from: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/.
3.     Cooper DB PP, Mahdy H. Oral Contraceptive Pills StatPearls StatPearls Publishing; 2022 [updated 2022 Nov 24; cited 2022. Available from: https://www.ncbi.nlm.nih.gov/books/NBK430882/.
4.     NHS. The progestogen-only pill -Your contraception guide NHS Website2021 [updated 02/02/2021. Available from: https://www.nhs.uk/conditions/contraception/the-pill-progestogen-only/.
5.     NHS. Combined pill -Your contraception guide NHS Website2020 [updated 01/07/2020. Available from: https://www.nhs.uk/conditions/contraception/combined-contraceptive-pill/.
6.     Hillard PA. Menstrual suppression: current perspectives. Int J Womens Health. 2014 Jun 23;6:631-7. doi: 10.2147/IJWH.S46680. PMID: 25018654; PMCID: PMC4075955.
7. Girum T, Wasie A. Return of fertility after discontinuation of contraception: a systematic review and meta-analysis. Contracept Reprod Med. 2018 Jul 23;3:9. doi: 10.1186/s40834-018-0064-y. PMID: 30062044; PMCID: PMC6055351.
8. Barnhart KT, Schreiber CA. Return to fertility following discontinuation of oral contraceptives. Fertility and Sterility. 2009;91(3):659-63. 
9. NHS. Your contraception guide NHS Website 2021 [updated 17/03/2021]. Available from: https://www.nhs.uk/conditions/contraception/?tabname=getting-started.