Background Unplanned pregnancy rates in South Africa are high. Effective use of contraception is therefore an essential public health intervention to prevent unplanned pregnancies. This study describes contraception use and its impact on pregnancy in women participating in HIV prevention research and its implications for public health practice.

Method A secondary analysis of sociodemographic, behavioural, contraception use, and pregnancy incidence data was conducted amongst women participating in the Microbicides Development Programme (MDP) 301 trial conducted in Durban, South Africa. Log-rank tests were carried out to compare the pregnancy incidence between women who reported use of injectable contraceptive methods compared to women using oral contraceptive pills, using condoms and other methods (intrauterine device, traditional methods and natural methods). The effect of types of contraceptives on pregnancy incidence was assessed using Cox proportional hazards regression models.

Results Of the 2018 women enrolled, injectable contraception was the most commonly used method (52%) compared to pills, condoms for pregnancy prevention and other methods. Injectable contraception use was associated with lower crude pregnancy incidence of 4.4 per 100 woman-years [95% confidence interval (95% CI 3.3–5.9)] compared to women using pills [19.3 per 100 woman-years (95% CI 13.3–28.0)], condoms [19.7 per 100 woman-years (95% CI 16.3–23.6)] and other methods [11.5 per 100 woman-years (95% CI 7.5–17.6)]. This effect remained significant when adjusted for age, level of education, condom use at last sex act [hazard ratio 0.27, (95% CI 0.16–0.47, p<0.001)].

Conclusion Injectable contraception offered a high level of protection against pregnancies among women in Durban.

Trial registration number ISRCTN64716212.