Contraceptive Effectiveness

Contraceptive effectiveness is the ability of a method to prevent pregnancy in real life. It is expressed as the percentage of women who become pregnant during the first year of use. Perfect use refers to consistent and correct use every time; typical use reflects real-world use including missed pills, late injections, incorrect technique and inconsistent use. The gap between the two shows how much a method depends on the user.

Typical and perfect use pregnancy rates in the first year, out of 100 women.

Efficacy versus effectiveness. Efficacy is the perfect use rate under ideal conditions. Effectiveness is the typical use rate in the population. Both are needed for honest counselling.

Factors that reduce typical use effectiveness.

• User-dependent schedules: daily pills, weekly patch, monthly ring, late injections.

• Adolescence and young age, high coital frequency and high underlying fertility.

• Stockouts, missed appointments and distance from facilities.

• Interacting drugs such as rifampicin and efavirenz.

• Vomiting, severe diarrhoea and malabsorption after bariatric surgery for oral methods.

Continuation. One-year continuation is highest with IUDs and implants and lowest with short-acting methods. Discontinuation without switching is a major contributor to unintended pregnancy.

Body weight. Implants, IUDs and DMPA keep their effectiveness at all weights. The patch is less effective at weight over 90 kg. Levonorgestrel emergency pills are less effective in women with a body mass index of 30 kg/m2 or more.

Communicating effectiveness.

• Use natural frequencies: "about 7 in 100 women using the pill typically become pregnant in a year, compared with fewer than 1 in 100 with an implant".

• Contrast typical and perfect use for user-dependent methods, and explain what closes the gap: reminders, extended supply, self-injection and clear missed-use rules.

• Avoid labelling any method as "safe" or "unsafe" in isolation; frame effectiveness against the patient's goals.

Effectiveness over time. Cumulative failure rises with years of use for user-dependent methods; over 10 years, about half of typical pill users may experience a pregnancy, while failure with an IUD or implant remains low.

Supporting adherence. Offer multi-month supply of pills (3 to 13 packs), DMPA-SC self-injection, reminder systems and a clear plan for late or missed doses at every visit.

Common errors.

• Quoting perfect use rates for user-dependent methods.

• Presenting fertility awareness methods as a single group; failure rates differ widely by method and by user.

• Ignoring the denominator: rates are for 100 women in the first year, not for each act of intercourse.

Contraceptive failure. Any pregnancy during contraceptive use needs prompt assessment of gestation and location. Pregnancy with an IUD in place carries a higher proportion of ectopic pregnancy.

Referral and escalation. Refer patients with repeated failures, suspected interaction, or failure of a long-acting method for evaluation of device position, product integrity or interacting medicines.