Contraception

Methods of Contraception: Success and Failure of Contraception

The Pearl Index
In order to arrive at some reliable figures about the success or failure of contraceptive methods, one needs a large sample, and for this 1200 months of contraceptive use are considered sufficient. In practice: One follows 100 women who have regular vaginal intercourse and who use a certain method of contraception. After 12 months one counts the unwanted pregnancies within this group. If two of the women have become pregnant in that one year, the failure rate for this particular method, i.e. the Pearl index, is 2. If five of the women have become pregnant, the Pearl index is 5. In other words: The higher the Pearl index, the greater the failure rate. Conversely: The lower the Pearl index, the more effective is the contraceptive method in question.
However, the failure rates measured by the Pearl index should not be considered absolute. First of all, different sources provide different index numbers. Some are simply provided by the industry, others are estimates with various ranges. Still, there is usually some agreement on the overall ranking. In any case, contraception often fails not because the method is ineffective, but because it is not used properly. For these and other reasons, the Pearl index can never be more than a guide post, a helpful hint, a piece of general information. Nevertheless, when seen in this light, it has its uses as one among several factors when individuals or couples select a contraceptive method for themselves.