Birth control failure rates - the Pearl Index explained
Medically reviewed by Drugs.com. Last updated on Dec 9, 2024.
The Pearl Index, or Pearl Rate, is a common technique used in clinical trials and real-world settings to help estimate the efficacy or effectiveness of birth control methods.
Clinical trials are commonly used to estimate the efficacy of a method - that is, how well it works when used correctly and consistently and the directions for use are followed (perfect use). Information from a real-world setting is used to estimate the effectiveness of a treatment - this is how well it works when it’s sometimes used incorrectly or inconsistently (typical use).
The Pearl Index was developed by Raymond Pearl in 1934 and has been used for over eighty years. It calculates the number of contraceptive failures for a birth control method per 100 woman years (HWY) of use. The following formula is used to calculate the Pearl Index:
| Pearl Index = (Number of pregnancies x 12) x 100 / (Number of women in the study x Duration of study in months) |
What do typical use and perfect use mean?
Some birth control methods are more effective - have lower failure rates - than others. How effective each birth control method is, is also affected by how well it is used and so the Pearl Index for ‘typical-use’ and ‘perfect-use’ for a particular method may both be reported.
Typical use describes how a birth control method is commonly used in a real-world setting. Failure rates associated with typical use take into account, for example, women and couples that have:
- Used a method correctly and consistently
- Used a method correctly but not used it each time they have had sexual intercourse
- Sometimes used a method incorrectly
- Sometimes used a method inconsistently
- Followed the wrong instructions on how to use the method correctly
Perfect use better describes how a birth control method is used in a clinical trial setting. When use is monitored and overseen by doctors, a birth control method is more likely to be used correctly and more consistently.
Perfect use is an estimate that includes results from women that have used the method correctly and consistently.
How effective a birth control method is can also be impacted by factors other than correct and consistent use such as:
- Frequency of intercourse - this is one of the most important determinants of pregnancy risk. Among women who used a diaphragm every time they had sexual intercourse, those who had intercourse more frequently were more likely to get pregnant, according to the results of a clinical trial.
- Age of woman - as a woman gets older they are less likely to become pregnant.
- Fertility - some women are more fertile than others and fall pregnant more quickly.
- Medications or other health conditions - some medications can make certain methods of contraception less effective. Various health conditions can also impact what contraceptive methods a woman can choose.
How effective are different forms of contraception?
The contraceptive methods with higher Pearl Index numbers are the least effective and are typically the methods that require daily usage, for example, the pill.
The table below reports the perfect-use and typical-use failure rates for different birth control methods - that is the number of pregnancies that would be expected to occur if 100 women used a particular birth control method for a year.
The table also shows the reported efficacy for each method - that is the percentage of women who would not be expected to get pregnant if they used the method of contraception correctly and consistently for a year.
Related questions
- Can antibiotics delay or make your period late?
- What are my birth control options and how effective are they?
- How long do antibiotics affect birth control?
Birth control method failure rates and efficacy
| Contraceptive Method | Perfect use failure rate | Typical use failure rate | Efficacy (%) perfect use |
| Progestin intrauterine device (IUD) | 0.5 | 0.7 | >99% |
| Copper IUD | 0.6 | 0.8 | >99% |
| Combined oral contraceptive - the pill | 0.3 | 7 | >99% |
| Progestin only pill - mini-pills | 0.3 | 7 | >99% |
| Progestogen only Injection | 0.2 | 4 | >99% |
| Monthly injection or combined injectable contraceptive (CIC) | 0.05 | 3 | >99% |
| Vaginal ring | 0.3 | 7 | >99% |
| Contraceptive patch | 0.3 | 7 | >99% |
| Hormone implant or rod | 0.1 | 0.1 | >99% |
| Sponge |
9 - women that have never had children 20 - women that have had children |
12 - women that have never had children 24 - women that have had children |
80 - 91% |
| Diaphragm | 4-8 | 12 | 92 - 96% |
| Male condom | 2 | 13 | 98% |
| Female condom | 5 | 21 | 95% |
| Cervical cap | 4-8 | 17-23 | 92 - 96% |
| Chemical contraceptives - spermicides | 18 | 28 | 82% |
| Withdrawal (Coitus interruptus) | 4 | 20 | 96% |
| No contraception | 85 | 85 | 15% |
| Sterilisation of the woman | 0.5 | 0.5 | >99% |
| Sterilisation of the man | 0.1 | 0.15 | >99% |
| Calendar method | unknown | 15-24 |
What are life table methods?
Life table methods or decrement tables are also used to calculate the probability of a birth control method failing and are an alternative to the Pearl Index.
Life table methods look to overcome one of the downsides of the Pearl Index, which is that it assumes a constant birth control failure rate over time, but this isn’t typically the case. Especially fertile couples may get pregnant sooner than others and so may not be included in a final analysis to determine a failure rate. The effectiveness of a birth control method can also improve over time as people become more experienced with how to use it. Both of these factors can affect the estimated failure rate at different points in time.
Life table methods are able to estimate the failure rate of a birth control method for each month of use, as well as over a longer period of time such as 12 months. They are able to eliminate the time-related biases that affect the Pearl Index method.
References
- Potter, Robert G. Application of Life Table Techniques to Measurement of Contraceptive Effectiveness. Demography, vol. 3, no. 2, 1966, pp. 297–304. JSTOR, [Accessed 17 December, 2020.]
- World Health Organisation (WHO). Family planning/contraception methods. 22 June 2020. Available at: https://www.who.int/news-room/fact-sheets/detail/family-planning-contraception [Accessed 17 December, 2020.]
- Trussell J. Contraceptive failure in the United States. Contraception. 2011;83(5):397-404. doi:10.1016/j.contraception.2011.01.021 [accessed 17 Dec. 2020]
- Centers for disease control and prevention (CDC). Reproductive health. Contraception. 13 August, 2020. Available at: https://www.cdc.gov/reproductivehealth/contraception/index.htm [Accessed17 December, 2020]
- Natural cycles. Birth control effectiveness explained. 31 March, 2020. Available at: https://www.naturalcycles.com/cyclematters/birth-control-effectiveness-explained [Accessd 17 December, 2020]
- Farley TM. Life-table methods for contraceptive research. Stat Med. 1986;5(5):475-489. doi:10.1002/sim.4780050512
- U.S. Food and Drug Administration (FDA). Birth control chart. 11 February, 2020. Available at: https://www.fda.gov/consumers/free-publications-women/birth-control-chart [Accessed 17 December, 2020]
- Trussell, J, Glob. libr. women's med., (ISSN: 1756-2228) 2014; doi 10.3843/glowm.10375. [Accessed 17 December, 2020]
- Guttmacher Institute. Contraceptive effectiveness in the United States. April 2020. Available at: https://www.guttmacher.org/fact-sheet/contraceptive-effectiveness-united-states [Accessed 17 December, 2020]
- National Health Service U.K (NHS). How effective is contraception at preventing pregnancy? 17 April, 2020. Available at: https://www.nhs.uk/conditions/contraception/how-effective-contraception/ [Accessed 17 December, 2020]
- Mclure Z. Failure rates of contraceptive methods. Fam Plann Inf Serv. 1981;1(6):59-61. https://pubmed.ncbi.nlm.nih.gov/12263458/
- Germán Rodríguez, Princeton University. Demographic Methods. Life table analysis of contraceptive use.
- Schoen, Robert. Constructing Increment-Decrement Life Tables. Demography, vol. 12, no. 2, 1975, pp. 313–324. JSTOR. [Accessed 17 December, 2020.]
Read next
What should I expect after a Mirena IUD removal?
It is typically less painful to get your Mirena IUD removed than it is to get it inserted but for a few hours or days after the removal you may experience some light pain and bleeding, cramping, or feel dizzy. An IUD removal typically takes about 5 minutes, but you should allow 20 minutes for the whole appointment. Consider eating something before your appointment so you are less likely to feel dizzy, and take some painkillers, such as acetaminophen or ibuprofen one hour before to help with any pain. Most women can go straight back to work or study or usual activities after the IUD is taken out. Continue reading
What are the signs that Plan B didn't work?
If you have had to take Plan B as an emergency contraceptive, you may be wondering if it has worked. While research shows it has an efficacy rate of 89% when used correctly, how easy is it to tell if it has worked for you? The easiest way to know that it hasn’t worked are a missed period and a positive pregnancy test. Continue reading
Can you have sex after taking Plan B?
While you can have sex after taking Plan B One-Step, it is important to use a back-up method to prevent pregnancy from occurring.
Continue readingSee also:
Related medical questions
- Why are my nipples sore? 7 possible causes
- Do antibiotics affect the morning after pill?
- What are the side effects of IUDs?
- When do I start taking Diane pills for birth control?
- What's the weight limit for Plan B?
- Is it normal to have discharge on Depo shot?
- Mirena, Kyleena, Skyla & Liletta - What's the difference?
- Can Kyleena cause weight gain?
- After having a Kyleena insertion, how long do I have to wait to have intercourse?
- When will I get my first period after inserting the Implanon implant?
- Is this normal - spotting and stomach cramps from starting birth control pills?
- Can Kyleena cause acne?
- Twirla vs Xulane: How do they compare?
- ParaGard vs Mirena: What's the difference?
- Is it okay to take Vyvanse while on birth control?
- How do I use Opill birth control?
- Can Kyleena make your period stop?
- Opill OTC vs other birth control pills: What's the difference?
- Can you get pregnant with an IUD?
- Can I get pregnant the first time I have sex?
- Should I stop using Xulane if I get a rash?
- How much does Opill cost?
- Can you start the Xulane patch anytime?
- Can you get pregnant on the Xulane patch?
Related support groups
- Birth Control (467 questions, 3,681 members)
- Contraception (237 questions, 1,063 members)