Back to basics: How do birth control pills work?

A symbol of liberation with a side-salad of second-wave feminism, ‘I’m on the pill ’, has become one of the modern era's most ubiquitous statements. 

Today, in the United States, 5.8 million women of childbearing age use oral contraceptives, and ‘the pill’ is widely known as one of the world’s most successful (and iconic) drugs ever developed. 

But despite our omnipresent pill-popping, how many of us actually know the answer to the very simple question, ‘how do birth control pills work?’

They may be small, but they certainly are mighty–let’s take a look at what those wonder-pills are actually doing to our bodies...

Stopping the sperm meets egg moment

After we ovulate each month and an egg is released from our ovaries, we enter what’s known as our ‘fertile window’. Generally, this is about a week in which, if a ‘sperm meets egg moment’ happens (you know it) we might get a bun in the oven. 

Birth control pills work with one simple aim in mind–stopping this sperm meets egg union from taking place. 

They do this by changing the levels of sex hormones in our systems, which induces a variety of bodily changes that keep our eggs at bay and those sperm away. 

But to understand exactly how birth control pills work, first, we need to understand exactly what birth control pills are.

Synthetic sex hormones

Birth control pills are a type of hormonal contraceptive. But what many of us don’t realize is that rather than containing ‘natural sex hormones’, most hormonal contraceptives contain what’s known as ‘synthetic sex hormones’ (with the caveat of those that contain bioidentical estrogen). 

Essentially, these are ‘(wo)man-made’, versions of the real deals. 

Synthetic sex hormones mimic the behavior and effects of our natural sex hormones–estrogen and progesterone–which fluctuate in a fine balance each month(1). Birth control pills intentionally disrupt this balance, suppressing our menstrual cycles with different doses of synthetic sex hormones(1).

This isn’t as scary as it sounds. The whole purpose of the pill is to prevent what our natural sex hormones are trying to achieve– getting our eggs fertilized and then fostering the resulting fetus.  

So, it’s really no surprise that the natural way hits the highway. 

Different birth control pills work in different ways depending on which kinds of synthetic hormones they contain. Let’s take a look at the two main types, Progestin-only pills (POPs) and Combined Oral Contraceptives. 

How do POPs work?

Progestin-only pills (POPs), sometimes referred to as mini-pills, only contain progestins. These are synthetic versions of the hormone progesterone. 

Progesterone is the dominant hormone in the latter half of our menstrual cycles and its main job is to prepare us for, and support our bodies, throughout pregnancy(2). If the sperm meets egg moment happens, instead of plummeting like it normally does (when we aren’t pregnancy pending), progesterone levels will stay at a steady high—pressing pause on our periods(3). 

This is because high progesterone levels suppress the production of our follicle-stimulating hormone (FSH) and our luteinizing hormone (LH)—the main players involved in getting our eggs ready for ovulation(3). No FSH or LH means no more eggs are called to arms, which means no ovulation.

High progesterone levels also cause our cervical mucus to thicken(4)(yep, we are talking about that wet patch in our knickers). This thick sticky mucus is notoriously hard for sperm to swim through, and acts as a natural roadblock at the entrance of our wombs. 

By keeping our progestin levels constant, POPs work by effectively ‘mimicking’ all of these effects of high progesterone levels—suppressing ovulation (eggs at bay) and thickening our cervical mucus (sperm away). 

However, here’s the catch. POPs containing older forms of progestin (like levonorgestrel) do not always completely suppress ovulation(5,6). But most modern POPs will contain a newer form of progestin (desogestrel) which has been found to suppress ovulation in 97% of cycles(5,7).

POPs are monophasic, meaning every pill in the pack contains the same amount of progestin, keeping it at a consistently high, steady level throughout the month. POPs are taken back-to-back, without a break between pill packs—there’s no need for a withdrawal bleed (fake period) and you may not bleed at all—but it’s also very common to experience breakthrough bleeding or irregular spotting.

When should I take a POP?

Usually, progestin-only contraceptives are prescribed when estrogen-containing contraceptives (CHC’s/COCs) aren’t suitable. For example if:

• You’re currently breastfeeding,

• Smoke and are 35 years of age or older

• Take any medications that may interfere with CHC’s(8–10).

We also should be mindful of our mental health when making choices about our birth control, as POPs are more commonly associated with adverse mood side-effects.

How do COCs work?

COCs, or Combined Oral Contraceptives, contain a combination of progestins and estrogens. Some COCs contain synthetic estrogens, but some more modern pills use bioidentical estrogens (a fancy way of saying they are naturally occurring). 

The progestins in COCs work the same way as they do in POPs—mimicking pregnancy levels of progesterone to suppress ovulation and thicken our cervical mucus(4). But it's the synthetic estrogen element in COCs that’s the showstopper here. Synthetic estrogens bolster progestins to completely suppress ovulation and increase the stability of our uterine (uterus) lining, which reduces breakthrough bleeding or spotting. 

This is why when taking COCs, over POPs, you are less likely to get any of those random bits of bleeding. 

COCs can be monophasic (just like POPs) but can also come in varying doses depending on the pill. You may be prescribed a biphasic, a triphasic or a quadriphasic pill. Essentially, this jumbled jargon means the amounts of synthetic estrogens and progestins in each pill change throughout the month to simulate a more ‘natural’ menstrual cycle.

When should I take a COC?

COCs are the most commonly prescribed oral contraceptive and usually a fairly popular choice because of the reduced breakthrough bleeding and the synthetic estrogen element which has lots of health benefits. But COCs aren’t suitable for everyone and come in many different shapes and sizes, so it’s important to figure out which kind of COC is right for you.

How do I know which pill type is right for me?

So there you have it, a whistle stop tour of what those wonder pills are up to behind the scenes. 

We’ve covered the clinical categories of POPs and COCs, but within these there are 100’s of different types of pill brands, all containing different forms and doses of synthetic estrogens and progestins. 

So how on earth do we know which of these birth control pills are going to be right for us? 

Luckily, that’s where we come in. 

At Tuune, we can help you find your perfect pill–one that’s right for you, your body and your hormones. Say goodbye to unforeseen side-effects, uncertainty and sub-par support. 

Tuune 360 takes a complete deep-dive into your medical history, your mental health, your physical health and your goals for using birth control. From there, we can recommend birth control options that are totally in tune with you. 

When our birth control is concerned, it’s soooo important to be supported with all of the right information, so we can make informed choices about our own health. 

Editor's note: Check out our article ‘Is it okay to skip my period on the pill’ for a 101 on how to use birth control pills, including everything you need to know about different regimens and finding the right one for you. 

Written by

Annalisa Hayes (she/her)

Copywriter at Tuune, Annalisa has worked for various pioneering health-tech startups and healthcare companies with purpose-led missions. Driven by empowering people to take control of their health, she helps make the science behind hormones accessible for our community, so they can make clued-up choices about their healthcare.

Medically reviewed by

Dr. Laura Jogineau

Gynaecologist, with a Masters in Clinical Medicine, Laura has lead public and private hospitals in Spain and was previously clinical Lead at Zava. Combining clinical practice with research, she regularly publishes research papers with her multidisciplinary group. Laura is passionate about women and people with cycles’ health and making healthcare accessible to everyone.

Scientifically researched by

Hannah Durrant (she/her)

Hannah is a Biomedical Content Writer at Tuune, with a BSc in Biomedical Sciences from University College London. She is passionate about bringing together the scientific community and the general public by disseminating modern science via digestible, engaging and thought-provoking content.