Pregnancy risk among oral contraceptive pill, injectable contraceptive, and condom users in Uganda, zimbabwe, and Thailand

Obstet Gynecol. 2007 Nov;110(5):1003-9. doi: 10.1097/01.AOG.0000268804.98744.2b.


Objective: To estimate the probability of pregnancy for oral contraceptive pill (OCP), injectable contraceptive, and condom users in Uganda, Thailand, and Zimbabwe.

Methods: This study is a secondary analysis of 5,224 women who participated in a prospective study evaluating the association between hormonal contraception and human immunodeficiency virus (HIV) acquisition.

Results: The overall 12-month cumulative probability of pregnancy of injectable contraceptive users was 0.6% (95% confidence interval [CI] 0.3-1.0), with similar risks in Uganda (0.3%, 95% CI 0-0.7), Thailand (0.6%, 95% CI 0-1.2), and Zimbabwe (1.0%, 95% CI 0.3-1.7). The 12-month cumulative probability of pregnancy for OCP users was 9.5% (95% CI 8.1-11.0%), with similar risks of pregnancy in Uganda and Zimbabwe (14.6%, 95% CI 11.7-17.4; and 10.2%, 95% CI 8.0-12.5, respectively) but substantially lower risk in Thailand (0.5%, 95% CI 0-1.2). The overall 12-month cumulative probability of pregnancy for women intending to use a given method at baseline was 2.0% (95% CI 1.4-2.6%) for injectable contraceptives, 15.7% (95% CI 14.1-17.3%) for OCPs, and 25.8% (95% CI 23.2-28.4) for condoms. Women in Thailand experienced lower pregnancy risk with condoms (18.4%, 95% CI 11.1-25.7) than in Uganda (29.5%, 95% CI 25.7-33.4), and Zimbabwe (23.3%, 95% CI 19.4-27.2).

Conclusion: The overall risk of pregnancy for injectable contraceptive users was substantially lower than for oral contraceptive pill users. However, Thai participants had similarly low cumulative pregnancy probabilities for both methods. Women receiving contraceptive counseling should be informed that their experience with a given method may differ from the average or typical-use pregnancy rates often discussed during contraceptive counseling. Tailored counseling is necessary for women to make informed choices.

Level of evidence: II.

Publication types

  • Comment
  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adolescent
  • Adult
  • Condoms*
  • Contraceptive Agents, Female / administration & dosage*
  • Contraceptives, Oral, Combined / therapeutic use*
  • Counseling
  • Female
  • Humans
  • Injections, Intramuscular
  • Kaplan-Meier Estimate
  • Medroxyprogesterone Acetate / administration & dosage*
  • Pregnancy
  • Pregnancy Rate*
  • Prospective Studies
  • Thailand
  • Treatment Outcome
  • Uganda
  • Zimbabwe


  • Contraceptive Agents, Female
  • Contraceptives, Oral, Combined
  • Medroxyprogesterone Acetate