The Pearl Index was developed to compare the different methods - ranging from sterilisation to "watchfulness" - and to offer a decision-making aid. It allows a statement to be made about how safe a contraceptive method is.
The Pearl Index is calculated on the basis of 100 women of childbearing age who use the contraceptive method for one year. A Pearl index of 0 would therefore mean that no woman became pregnant during this period, the contraceptive is perfect.
A Pearl index of 1 would mean that one in 100 women became pregnant despite contraception, while a Pearl index of 0.1 would mean that one in 1000 women became pregnant within one year. In summary, the lower the Pearl index, the safer the method of contraception.
By way of comparison, in the case of regular sexual intercourse without any form of contraception, the Pearl Index for young healthy women aged 20 is around 80 to 90. The number of pregnancies that occur without the use of contraceptive measures decreases steadily with increasing age.
The total number of pregnancies that occur when using a particular contraceptive method is known as the "Practical Pearl Index". It indicates which method works best in everyday life and reflects real life, as it can also lead to pregnancies in the event of application errors. The "Theoretical Pearl Index", on the other hand, describes how well the method works with error-free application.
If the values are approximately in the same range (e.g. spiral), then the method is easy to use and you can do little wrong. However, if the values are far apart (e.g. condom), it is important for effective contraceptive protection to carry out a constant and consistent application.
Sterilisation, hormone implants, hormone and copper spirals and hormonal methods such as the pill and three-month syringe are among the safest contraceptive methods. Barrier methods such as condoms offer slightly less safety due to frequent user errors. Natural contraceptive methods (e.g. temperature measurement) are generally not recommended due to the high Pearl index.
IMPORTANT: The Pearl Index only allows a statement to be made about the prevention of unwanted pregnancies. The index does not provide any information about the probability of infection with sexually transmitted diseases. Only the condom offers effective protection against sexually transmitted diseases such as HIV. A protective vaccination is also available to protect against carcinogenic papilloma viruses.
The safety of a contraceptive method depends crucially on its correct use. For example, if the pill is taken irregularly or a condom is used improperly, optimal contraceptive protection is no longer provided. These so-called application errors can significantly impair the safety of a contraceptive.
If the contraceptive fails despite optimal use, this is referred to as a method error. This often unclear distinction between the two types of "sources of error" is one reason why the literature sometimes contains very different data on the pearl index of a contraceptive - depending on whether application errors were included in the investigation or not.
Written by Mark Riegel, MD
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