Pearl index and safety in contraception

The "Pearl Index" is used by default to rate various contraceptives on their safety and effectiveness. But what can this index tell us, how is it calculated and are the calculations really that reliable? We answer the most important questions about the Pearl Index.

What is the Pearl Index?

The Pearl Index tries to make a statement about the safety of a contraceptive. It interprets the data collected for a specific contraceptive in such a way that it can be shown how many of 100 users became pregnant despite using a certain contraceptive. 

A Pearl Index of 2 means that 2 out of 100 women became pregnant despite using a specific contraceptive. There is then talk of a “failure rate” of 2%.

The Pearl Index is also calculated from method reliability and application reliability. Method reliability indicates how many pregnancies occur despite the contraceptive being used correctly. App security is more practical, on the other hand, and shows how many women, taking into account app errors that occur in everyday life, get pregnant unintentionally. If method and application safety are close together, it can be assumed that the contraceptive is generally safe.

How is the Pearl Index measured or calculated?

The lower the Pearl Index, the greater the protection of a contraceptive. But how can the Pearl Index be calculated?

The calculation is based on 1,200 months of use (100 women in a year) of a contraceptive. Thus, the index finds out how many of those 100 women became pregnant within a year using each contraceptive.

For example, with a Pearl Index of 2, 2 out of 100 women will become pregnant in one year. For example, if the Pearl Index is 0.05, one in 2,000 women will become pregnant within a year.

Pearl index: strengths and weaknesses

The Pearl Index is a good and easy to calculate reference point for anyone who prefers to play it safe when choosing their contraceptives and for whom the comparison can be made easier with the help of this reference value.

However, it is very important that the effectiveness of a contraceptive always depends on its correct use. Unfortunately, the Pearl Index (PI) also has some weaknesses, which we will list below.

The PI does not take into account the increasing probability of becoming pregnant over time. Also, some women have more sex than others. Diseases such as gastrointestinal can also reduce the effectiveness of hormonal contraceptives.

In addition, there is the already mentioned problem in the use of different contraceptives (link on the contraception category page). While some are very easy to use, others become much more complex. 

To counteract this problem, the "Life Table Analysis" was designed. Here not only the frequency of intercourse and individual application errors are taken into account, but also information is given on women who did not become pregnant. This makes it possible to specify a percentage of protected women for individual months.

The Pearl Index should primarily serve as a guide. Nor does it give information about the risk of contracting sexually transmitted diseases. Depending on your life situation and individual factors, it is important to speak with your gynecologist.

Pearl index in hormonal methods

With hormonal birth control methods like the pill or the hormonal patch, the first thing to keep in mind is that it is not a barrier method. Hormonal methods do not guarantee protection against sexual diseases such as genital herpes, chlamydia or HIV.

The contraceptive method most used by women is the pill. About 31% of women take them and trust their effectiveness. In most cases, the Pearl Index of the pill is between 0.1 and 0.9. This also makes it one of the safest birth control methods, according to the IP.

However, this security also comes at a high price, because the list of side effects seems endless. Weight gain, mood swings, breast tenderness, headaches, or nausea are just a few of the many possible side effects.

Alternatives to the Pearl Index

One thing is clear: the Pearl Index can be of great help in deciding how to prevent it. However, it is also susceptible to disturbances that affect the result. Forgetfulness, illness, or drug interactions can affect many birth control methods, making them less effective.

For this reason, the Pearl Index can be supplemented with two additional groups and made more secure. For example, by examining unintended pregnancies in 100 women over a twelve-month period, the Pearl Index can be divided into application safety (practical PI) and method safety (theoretical PI).

The safety of use provides information on the typical and daily use of a contraceptive. Factors such as forgetting to take medication, gastrointestinal diseases or errors in its use are taken into account.

On the other hand, there is the reliability of the method. It assumes a completely correct use of the contraceptive method and shows a value that theoretically indicates the safety of a contraceptive method.

If you look at these two values in a common context, you can conclude: the closer the method and the reliability of the application, the more accurate the Pearl Index is. The lower the Pearl Index, the safer the birth control method.

Pearl index in natural methods

The Natural Family Planning (NFP) wants to do without the addition of hormones or chemical agents. Unfortunately, natural contraception is still often assumed to be extremely complex and unsafe or even require operations. However, these assumptions are incorrect.

The safest method of natural contraception is Symptothermal Method, according to which it also evaluates and makes use of the trackle contraceptive monitor. With so-called perfect use, ie no vaginal intercourse during the fertile period, it has a Pearl Index of 0.4 in terms of method reliability and is therefore one of the most reliable family planning methods.

Regarding the safety of the application, also known as "typical use", which also includes the incorrect application of the method, the Symptothermal method has a Pearl Index of 1.8.

The Symptothermal Method You can determine your fertile window by looking at changes in basal body temperature and cervical mucus. trackle simplifies this method, eradicates its susceptibility to errors and makes it more digital. trackle It is worn overnight while sleeping and records core body temperature throughout the night. When you wake up the next morning, trackle sends the temperature data to your app from its station. For the evaluation, trackle only needs your cervical mucus information. Clever! This is what makes the Symptothermal Method a fun and safe method of applying.

The Pearl Index: a good guide

The Pearl Index is a good way to compare birth control methods and their safety. The Pearl Index allows you to orient yourself among the variety of contraceptive methods and make an initial selection. However, it is always advisable to keep in mind that: no matter what method you use for contraception, safety and reliability give you correct use. That's why you should sit down with your gynecologist for all contraception-related questions and get detailed advice on finding the method that's right for you. If you have any questions about how to obtain contraception with trackle, write to us at – We look forward to your questions!



Diedrich, K. et al.: Gynecology and obstetrics. 2nd Edition. Heidelberg. 2007Frank-Herrmann, P. et al.: Natural family planning today. Modern cycle knowledge for advice and application, 6th edition. Heidelberg, 2020