Effectiveness
The effectiveness of cervical caps, as of most forms of contraception, can be assessed two ways: method effectiveness and actual effectiveness. The method effectiveness is the proportion of couples correctly and consistently using the method who do not become pregnant. Actual effectiveness is the proportion of couples who intended that method as their sole form of birth control and do not become pregnant; it includes couples who sometimes use the method incorrectly, or sometimes not at all. Rates are generally presented for the first year of use. Most commonly the Pearl Index is used to calculate effectiveness rates, but some studies use decrement tables.
Contraceptive Technology reports that the method failure rate of the Prentif cervical cap with spermicide is 9% per year for nulliparous women (women who have never given birth), and 26% per year for parous women. The actual pregnancy rates among Prentif users vary depending on the population being studied, with yearly rates of 11% to 32% being reported.
Little data is available on the effectiveness of the Oves cap and Femcap. The Oves manufacturer cites one small study of 17 users. The Femcap website does not cite any data on the current version of the Femcap; but lists data for an older version which is no longer approved by the FDA.
The only effectiveness trial of Lea's Shield was too small to determine method effectiveness. The actual pregnancy rate was 15% per year. Of the women in the trial, 85% were parous (had given birth). The study authors estimate that for nulliparous women (those who have never given birth) the pregnancy rate in typical use may be lower, around 5% per year.
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