Birth control failure rates - the Pearl Index explained

The Pearl Index, or Pearl Rate, is a common technique used in clinical trials and real-world settings to help estimate the efficacy or effectiveness of birth control methods.

Clinical trials are commonly used to estimate the efficacy of a method - that is, how well it works when used correctly and consistently and the directions for use are followed (perfect use). Information from a real-world setting is used to estimate the effectiveness of a treatment - this is how well it works when it’s sometimes used incorrectly or inconsistently (typical use).

The Pearl Index was developed by Raymond Pearl in 1934 and has been used for over eighty years. It calculates the number of contraceptive failures for a birth control method per 100 woman years (HWY) of use. The following formula is used to calculate the Pearl Index:

What do typical use and perfect use mean?

Some birth control methods are more effective - have lower failure rates - than others. How effective each birth control method is, is also affected by how well it is used and so the Pearl Index for ‘typical-use’ and ‘perfect-use’ for a particular method may both be reported.

Typical use describes how a birth control method is commonly used in a real-world setting. Failure rates associated with typical use take into account, for example, women and couples that have:

• Used a method correctly and consistently
• Used a method correctly but not used it each time they have had sexual intercourse
• Sometimes used a method incorrectly
• Sometimes used a method inconsistently
• Followed the wrong instructions on how to use the method correctly

Perfect use better describes how a birth control method is used in a clinical trial setting. When use is monitored and overseen by doctors, a birth control method is more likely to be used correctly and more consistently.

Perfect use is an estimate that includes results from women that have used the method correctly and consistently.

How effective a birth control method is can also be impacted by factors other than correct and consistent use such as:

• Frequency of intercourse - this is one of the most important determinants of pregnancy risk. Among women who used a diaphragm every time they had sexual intercourse, those who had intercourse more frequently were more likely to get pregnant, according to the results of a clinical trial.
• Age of woman - as a woman gets older they are less likely to become pregnant.
• Fertility - some women are more fertile than others and fall pregnant more quickly.
• Medications or other health conditions - some medications can make certain methods of contraception less effective. Various health conditions can also impact what contraceptive methods a woman can choose.

How effective are different forms of contraception?

The contraceptive methods with higher Pearl Index numbers are the least effective and are typically the methods that require daily usage, for example, the pill.

The table below reports the perfect-use and typical-use failure rates for different birth control methods - that is the number of pregnancies that would be expected to occur if 100 women used a particular birth control method for a year.

The table also shows the reported efficacy for each method - that is the percentage of women who would not be expected to get pregnant if they used the method of contraception correctly and consistently for a year.

Birth control method failure rates and efficacy

What are life table methods?

Life table methods or decrement tables are also used to calculate the probability of a birth control method failing and are an alternative to the Pearl Index.

Life table methods look to overcome one of the downsides of the Pearl Index, which is that it assumes a constant birth control failure rate over time, but this isn’t typically the case. Especially fertile couples may get pregnant sooner than others and so may not be included in a final analysis to determine a failure rate. The effectiveness of a birth control method can also improve over time as people become more experienced with how to use it. Both of these factors can affect the estimated failure rate at different points in time.

Life table methods are able to estimate the failure rate of a birth control method for each month of use, as well as over a longer period of time such as 12 months. They are able to eliminate the time-related biases that affect the Pearl Index method.