Interpreting Birth Control Failure Rates

When choosing a contraceptive, it's important to factor in failure rates. These statistics, which refer to how often specific methods of birth control fail in preventing pregnancy, are one way to measure how effective a particular method is likely to be.

The concept of birth control failure rates may sound complicated, but in fact, it's based on simple and straightforward math. Once you understand how birth control failure rates are determined and what they mean, you'll be able to use them to make an informed decision about the best contraceptive method for you.

How Birth Control Failure Rates Are Determined

The birth control failure rate is the frequency with which a particular birth control rate fails (for this purpose, failure means that pregnancy is not prevented by the method). Failure rates are meant to be fairly reliable estimations of birth control effectiveness.

Often, 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 also can be affected by demographics, educational levels, culture, and the technique used to teach how to use the contraceptive method.

Failure rates are calculated for each birth control method based on the number of pregnancies that are prevented by using that contraceptive. This can be expressed as 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.

For example, what does it mean that condoms have a 2% to 15% failure rate? Another way to understand this is that condoms are 85% to 98% effective. The effectiveness rate is the opposite of the failure rate. Subtract the failure rate from 100, and that number is the birth control effectiveness rate. Condoms are 85% to 98% effective (meaning they have a failure rate of 2% to 15%).

This means that for every 100 women whose partners use condoms, from two to 15 of them 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 year. Failure rates refer to the number of pregnancies that take place when 100 women use that birth control method for one year.

Comparing Birth Control Failure Rates

Birth control failure rates usually refer to the number of people (out of 100) who use a birth control method and who will become pregnant during the first year of use. There are a number of factors that can affect failure rates, including experience with the method, the difficulty and effort associated with the method, and if the method is used correctly and consistently.

In practice, it appears failure rates tend to be higher during the first year a contraceptive is used. There are several reasons why failure rates may decrease after using a method for one year:

• The more experience you have using a birth control method, the more effective it becomes. The longer you use a method, the more comfortable and better skilled you become at 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.

The reason you may see a range in the birth control failure rates has to do with "typical use" vs. "perfect use." So, when the failure rates are presented in a range, the lower number represents perfect use and the higher number is for typical use.

• Typical use failure rates tend to 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 use contraception correctly.

• Perfect use failure rates reflect pregnancies that occurred with individuals even though they always used their contraception correctly and consistently.

Typical user failure rates tend to be higher than perfect 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.

These methods include:

• Fertility awareness method

• Condoms (both external and internal)

• Cervical caps

• Diaphragms

• Oral hormonal contraceptives

Sometimes, you will not see a range in failure rates. This means that typical use is equal to perfect use, as is the case with IUDs, surgeries (tubal ligation and vasectomy), and others.

Choosing Birth Control Based on Failure Rates

When comparing birth control methods, pay attention to whether the numbers refer to failure rates or effectiveness rates, as well as typical use or perfect use. Keep in mind that, regardless of the posted effectiveness or failure rate, other factors (in addition to user error or inconsistent use) can lower the effectiveness of birth control methods. These range from motivation to weight to medications you may be taking.

Also consider how often you have sex: If you know you'll be having sex frequently, it may be a wiser choice to use a more effective method to have the best chances of not getting pregnant.

Understanding how to interpret failure rates, knowing factors that could influence contraceptive effectiveness, evaluating your lifestyle and sexual behavior, and determining the level of effectiveness that is most acceptable to you can greatly help in the birth control decision you make. Consult with a healthcare provider for help in making a decision about what method will work for you and your lifestyle.