Is It Possible to Get Pregnant on the Pill?

Is it possible to get pregnant on the pill?


The pill is used daily by millions of women to protect against pregnancy. When it's used correctly, the pill is over 99% effective. This means that less than 1 in 100 women will get pregnant over the course of a year as a result of using the pill. There are factors that can affect this, however. 

How does the pill prevent pregnancy? 

The pill protects you against pregnancy in three ways. Firstly, it stops your ovaries from releasing an egg each month. This means that fertilization cannot occur. It also causes the mucus surrounding the cervix to thicken which makes it difficult for sperm to enter through the vagina. If both these circumstances fail, the hormones contained in the pill also thin the lining of the womb so that a fertilized egg cannot attach itself to the uterine wall. 

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What causes the pill to fail? 

Forgetting to take it

The most common reason for the contraception failing is by not taking it correctly. If you forget a pill it's important to carefully read the instructions about what to do as different pills contain varying amounts of hormones and some vary throughout the pack. To be on the safe side, you should use an extra type of contraception, like condoms, for 7 days after missing a pill. Always take the missed pill alongside the one you are due to take and make sure you take the rest as usual. 

The more pills you forget, or take incorrectly, the greater the risk of pregnancy. Also, be aware that if you miss a pill during the first week of your cycle, if you've had sex in the days leading up to this then you are also at risk of pregnancy. If you are ever worried or unsure about what to do, visit your GP or pharmacist for advice. You can still take the morning after pill if you need it. 


Having a stomach upset soon after you've taken the pill may mean that the pill won't work. This is because the hormones haven't had a chance to be absorbed by your body yet. As a general rule, vomiting within 3-4 hours of taking your pill means you can assume it won't be effective. Take another pill if you are able to. If not, or if your vomiting continues, treat this as a missed pill. You'll need to use another type of contraception, like condoms, until you have felt better for a full 7 days. Otherwise, you won't be protected against pregnancy. Diarrhea will not usually affect the pill unless it is severe. 


There are some forms of medication that interact with the pill, meaning it will not work properly. If you have HIV, Hepatitis C or epilepsy, many of the treatments prescribed for these conditions will cause the pill to be less effective. In addition to this, there are also certain antifungal treatments and two types of antibiotics called Rifampicin and Rifabutin and the herbal remedy St. John's wort. If you are taking any of these medications then the combined pill will not be suitable for you. Let your doctor know that you are taking the pill before being prescribed with any new medications. 

Is the mini pill as effective as the combined pill? 

The mini pill is just as effective as the combined pill. It only contains one type of hormone, progesterone. Just like the combined pill, the mini-pill is 99% effective with perfect use. However, the mini-pill needs to be taken on time every day and has less leeway than the combined pill. This means that with typical use, it’s probably only 91% effective because it must be taken within 3-12 hours depending on the type of progesterone-only pill. 

If I miss my period while I’m on the pill does this mean I’m pregnant? 

If you’re on the combined pill, your period will happen during the pill-free week before you start your new pack. Occasionally, you might not bleed at all. This does not necessarily mean you are pregnant. It’s more likely that the womb lining was thinner than usual so that there was nothing to shed at the end of your cycle. If you didn’t take your pills correctly or you’ve missed more than one period, then see your GP because you might be pregnant. 

The mini pill can cause your periods to be irregular and some women might not get a monthly bleed at all. This is normal and not a cause for concern. Again, if you have missed pills during the month or are concerned at all then visit your doctor or a pharmacist for advice. 

How will I know if I am pregnant? 

The only way to know for sure if you are pregnant or not is to take a pregnancy test. There are pregnancy tests readily available from pharmacies and shops for at-home use or you can go to a sexual health clinic or GP. There are signs and symptoms which you may also experience: 

  • Nausea 
  • Vomiting 
  • Breast tenderness 
  • Missed periods 
  • Light spotting or bleeding 
  • Breast swelling 
  • Tiredness
  • Mood swings 
  • Food cravings 
  • Frequent urination 
  • Lower back pain 
  • Constipation 
  • Flatulence 


NHS > The Combined Pill
NHS > The Progestogen-only pill 
Kid Spot > Early Pregnancy Symptoms

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