Factors associated with pregnancy in women taking part in a phase III microbocide trial in Johannesburg
Introduction This was a secondary data analysis of a prospective cohort of women enrolled in a phase III microbicide trial between October 2005 and August 2008. The study aimed to assess the pregnancy incidence rates and factors associated with pregnancy in women using barrier method and hormonal contraception, enrolled in the trial. Methods A total of 2508 participants were enrolled in the trial and followed up for up to 12 months. Of these 2437 were included in the pregnancy incidence analysis and 2171 participants were included in the multivariate analysis. Data on the main exposure, contraception, were collected by structured interview. The main outcome of interest was pregnancy, which was measured by detection of human chorionic gonadotrophin in urine using Quick Vue® test and confirmed by laboratory based testing. The incidence rate of pregnancy was calculated as number of pregnancies per 100 women years of follow up. Kaplan Meier Survival analysis was used to determine average time to first pregnancy. Univariate and multivariate analyses were conducted using Cox regression models to asses the factors associated with incident pregnancies. Data was analysed using Stata® version 10. Results A total of 2248 women years of follow up were recorded. A total of 238 pregnancies occurred resulting in pregnancy incidence of 11 per 100 women-years of follow up (95% CI: 9.32 to 12.02). The incidence of pregnancy increased with time in the study; 98 per 100 women years of follow up (95% CI: 85.09 to 112.35) in the last 3 months compared to 2 per 100 women-years of follow up (95% CI: 0.94 to 2.92) in the first 3 months of follow up. Older age and hormonal contraception use were significantly associated with a decreased risk of pregnancy. Women 35 years and older were 49% less likely to fall pregnant compared to those who were younger than 25 years, adjusted hazard ratio (AHR) 0.51(95% CI: 0.30 to 0.88, p=0.016). Women who used hormonal contraception had a reduced risk of falling pregnant AHR 0.66(95% CI: 0.46 to 0.94, p=0.02). There was no difference between the two types of hormonal contraception (injectable vs oral) with respect to pregnancy risk. Conclusion: The incidence of pregnancy increased with time in the study. Women who used hormonal contraception and who were older were less at risk of pregnancy. There was no significant difference in pregnancy risk by type of hormonal contraception (i.e. oral contraception vs injectable contraception) used.