Birth Control Effectiveness & the Pearl Index Explained

Birth control affects many of us throughout our lifetimes. One thing that’s important to consider when choosing a birth control is the effectiveness of the method. In this article we’ll take a look into the ways birth control effectiveness is worked out, how it’s measured and how your birth control can teach you more about your body.

Table showing hormonal and non-hormonal birth control and their typical use effectiveness

How effective is birth control?

Good question. There’s something we have to clear up before we go any further. No birth control is 100% effective. Whatever method you choose to use, there will still be a risk of pregnancy. However, some birth control methods are more effective than others and the good news is that using birth control can significantly reduce the risk of getting pregnant.

How can we measure birth control effectiveness?

Every woman is different and will have a different experience on birth control. However, it’s important we have a clinical measurement when we talk about birth control effectiveness. In the contraceptive landscape, we normally talk about two measures of effectiveness called typical use and perfect use.

What is typical use?

As well as method failure, typical use also incorporates user error. This includes:

  • The method being correctly used every time the couple have sex
  • The method is correctly used, but not used every time a couple has sex
  • The method is used, but sometimes incorrectly
  • The method is used incorrectly or sometimes not at all

As you can see, typical use provides room for error - hence why the failure rate is usually higher than perfect use. This is particularly true for those methods that require work from the user, for example, condoms have a typical use birth control effectiveness of 82%.

What is perfect use?

Without taking into account user error, perfect use looks at the method’s effectiveness in a perfect world - i.e. if it was used exactly as it was intended every time a couple have sex. There is still a small failure rate that applies if the method itself should stop working, but perfect use tends to be higher than typical use as it removes user error completely. For example, condoms have a perfect use effectiveness of 98%.

What is the Pearl Index?

The Pearl Index is defined as the number of pregnancies that happen for one method per 100 women over a year. So for example, the birth control pill has a typical use effectiveness of 93% and a Pearl Index of 7. So that means that seven women in one year will get pregnant from using the pill. The pearl index is usually a small number and can be used when talking about both typical use and perfect use. 

What are the most effective birth control methods?

The methods that have the highest effectiveness ratings, and the lowest Pearl Index, are those methods that don’t require much work from the user. For example, the copper coil has a very high perfect use effectiveness of >99%, and it’s typical use effectiveness is the same. That is because once the IUD is fitted, it doesn’t require any work at all from the user. The hormonal IUD and implant also have very high typical and perfect use effectiveness.

Other birth control options that do require daily commitment, for example the pill or Natural Cycles, have lower typical use effectiveness, because their continued and correct use depends on us. That might sound like more work, but we think it’s a good thing that women have access to a wide selection of birth control. This means we can all have more choice to find the best option for us. Whether our birth control is hormone-free, long acting or can prevent sexually transmitted infections are all considerations that will be less or more important to each of us.

How effective are different birth control methods?

Since every method is different and works in its own way, the effectiveness of methods can vary quite considerably. It’s worth knowing the effectiveness of different methods to understand how effective your method is in the wider contraceptive landscape. Below is a table of birth control effectiveness showing both typical use and perfect use for a range of common methods including both hormonal and non-hormonal birth control

Reference: Contraceptive Technology (Table 26 - 1). 21st Edition, 2018 —>
Source: Trussell J . Contraceptive Efficacy. In Hatcher R A, Trussell J, Nelson A L , Cates W, Kowal D, Policar M. Contraceptive Technology: New York NY: Ardent Media, 2018—>

Of course, these are just a number of contraceptive methods, there are many more options of birth control available. The World Health Organization provides a longer list of birth control methods and the numbers around typical use and perfect use - check it out here.

How likely am I to get pregnant using no birth control?

Using the Pearl Index, we can also understand the rate of pregnancy when no method is used. If we look at 100 women over the course of a year, who do not use birth control and are regularly having unprotected sex, then 85 of them will get pregnant according to the Pearl Index measurement. That’s a significantly higher number than those using recognized forms of birth control. 

It’s worth keeping in mind that the Pearl Index is a clinical measurement, and we are all different. Our fertility may vary from person to person and our ability to conceive starts to decrease as women get older. However, if you wish to prevent pregnancy, we recommend finding the best method of birth control that will fit your needs and your lifestyle.

Birth control can teach you more about your body 

Here at Natural Cycles, we’re committed to closing the knowledge gap when it comes to reproductive health. This means debunking myths, sharing facts and taking part in the wider conversation about our reproductive health.

Our birth control app is the first of its kind to be cleared by the FDA and it works by pairing the basal body temperature method with an algorithm that learns your unique cycle. The result is a birth control that’s tailored to you. With over one million registered users worldwide, Natural Cycles is hormone-free birth control that puts you in control.

Ready to get started?

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Written By

Jennifer Gray

A writer with a passion for women’s health, Jennifer Gray has years of experience writing about various reproductive health topics including birth control, planning pregnancy, women’s anatomy, and so much more.

Jack in a suit and tie holding a microphone and giving a presentation.

Scientifically Reviewed

Jack Pearson

With 10 years of experience working in the field of fertility, Dr. Jack Pearson is Natural Cycles’ in-house expert. As Medical Affairs Manager, he dedicates his time to conducting groundbreaking research and educating healthcare professionals.

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