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How Do You Interpret Birth Control Failure Rates?

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Updated February 02, 2012

How Do You Interpret Birth Control Failure Rates?

Birth Control Failure Rates

Photo © 2012 Dawn Stacey
Question: How Do You Interpret Birth Control Failure Rates?

"I am so confused about how to interpret failure rates. It is making it so difficult to figure out exactly how effective certain birth control is. For example, I read that the failure rate of condoms is 2-15% when 100 couples have sex regularly for a year. Does this mean that if a couple has sex 100 times in a year, using condoms each time, that there is a 2-15% chance that a pregnancy will occur? Also, why is there a range in failure rates for some birth control methods and not others?"

Answer:

In order to choose the best birth control method for you and to maximize its effectiveness, it is important that you understand how to interpret birth control failure rates. I would like to point out, however, that failure rates are meant to be fairly reliable estimations of birth control effectiveness – yet they are not absolute. Most failure rates are determined in clinical research studies with sample populations of participants. Theoretically, it is possible that different subject pools using the same birth control method can generate different failure rates. Researchers try to minimize this by using a large number of diverse participants. Failure rates, in research, can also be affected by demographics, educational levels, culture and the instruction technique that is used to teach how to use the contraceptive method.

That being said, this is a good question and is a concept that is often misunderstood. So, what does it mean when you read condoms have a 2-15% failure rate? Well, another way to understand this is that condoms are 85-98% effective. The effectiveness rate is the opposite of the failure rate... you basically subtract the failure rate from 100, and that number is the birth control effectiveness rate.

Failure rates are typically calculated for each birth control method based on the number of pregnancies that are avoided by using that contraceptive, or the difference between the number of pregnancies expected to occur if no method is used and the number expected to take place with that method. The proper way to interpret birth control effectiveness/failure rates is as follows:
Using condoms as an example –- Condoms are 85-98% effective (meaning they have a failure rate of 2-15%).

This means that: of every 100 women whose partners use condoms, 2 to 15 will become pregnant within the first year of use. So basically, the failure rate does not refer to how many times you have sex, it correlates the number of people (100) who use that method over the course of one the year.

The reason why you may see a range in the rates has to do with "typical use" vs. "perfect use":

  • Typical use failure rates tend to more represent how the average individual uses contraception. These rates apply to folks who became pregnant while not always using their contraception correctly and/or consistently. In reality, many people find it challenging to always and reliably use contraception correctly.
  • Perfect use failure rates reflect pregnancies that occurred with individuals even though they always use their contraception correctly and consistently.

Typical user failure rates tend to be higher than perfect use because, in a nutshell, the contraceptive is not being used perfectly. So, when the failure rates are presented in a range, the lower number represents perfect use and the higher number is for typical use. Birth control methods that require more for a person to do (i.e., remember to use, be inserted or put on a certain way, be used within a certain timeframe, etc.), tend to have higher failure rates because there is more room for error. Sometimes, you will not see a range in failure rates; this means that typical use is equal to perfect use. This tends to be the case for contraception that is inserted or performed by a doctor. Once this happens, there is virtually nothing for a person to do, so this eliminates typical user errors. Examples of contraceptives with equal perfect use and typical use failure rates are:

One last thing to keep in mind when it comes to birth control failure rates is that they usually refer to the number or women (out of 100) who use that birth control method who will become pregnant during the first year of use. In practice, it appears that the failure rates tend to be higher during the first year that you consistently use one particular contraceptive. There are several reasons why failure rates may decrease after using a method for one year. These include:

  • The more experience you have using a birth control method, the better its effectiveness. The longer you use a method, in theory, the more comfortable and better skilled you become at properly using it. This should help to reduce typical user errors.
  • Less motivated users may become pregnant and stop using contraception (leaving those who are still using a particular method after a year to be more serious and devoted users).
  • A woman’s fertility-level decreases with age, so with each year that passes, she is less likely to become pregnant.

Final Thoughts: failure rates are based on the number of people who use a given birth control method over the course of a year. The number of times you have sex during that year does not factor into these rates. However, the frequency that you have sex may be one of your criteria for choosing a birth control method. Basically, if you know that you will be having a lot of sex, it may be a wiser choice to use a more effective method to have the best chances of not getting pregnant. You should consider how detrimental an unplanned pregnancy would be. These are some of the considerations to take into account when comparing the failure rates of various contraceptive methods. You should also keep in mind that irrespective of the posted effectiveness/failure rate, other factors (in addition to user error or inconsistent use) can lower the effectiveness of certain birth control methods. These can range from your motivation to your weight to certain medications that you use.

When comparing birth control methods, make sure to pay attention to whether the posted numbers refer to failure rates or effectiveness rates as well as typical use or perfect use. You also should decide how reliable you need your contraception to be. Understanding how to interpret failure rates, knowing factors that could influence a contraceptive’s effectiveness, evaluating your lifestyle and sexual behavior, and determining the level of effectiveness that is most acceptable to you can greatly help in any birth control decision that you make.

Source:

Trussell J, Hatcher RA, Cates W, Stewart FH, Kost K. A guide to interpreting contraceptive efficacy studies. Obstet Gynecol. 1990; 76:558-67. Accessed via private subscription.

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