Eight observational studies were included in the review (8,900 patients, where reported): five case-control studies (6,658 patients) and three retrospective cohort studies (2,242 patients). Total number of patients per study ranged from 202 to 3,052 (where reported). All case-control studies had matched patients by age; two also matched according to medical record number or general practice. Two studies performed stratification. All of the studies had adjusted for covariates in multivariate analyses.
No statistically significant difference was found between users of progestin-only contraception versus non-users for risk of venous thromboembolic event (adjusted RR 1.03, 95% CI 0.76 to 1.39; eight studies). Low statistical heterogeneity was shown (Ι²=24%). A similar result, with no statistical heterogeneity (Ι²=0%), was shown when the analysis was re-performed using crude data (excluding one study for which this was unavailable).
There were no statistically significant differences in venous thromboembolic risk between users and non-users of progestin-only pills (RR 0.90, 95% CI 0.57 to 1.45; seven studies) or between users and non-users of a progestin intrauterine device (RR 0.61, 95% CI 0.24 to 1.53; two studies).
Risk of a venous thromboembolic event was statistically significantly higher for users of injectable progestin versus non-users (RR 2.67, 95% CI 1.29 to 5.53; two studies). No statistical heterogeneity was indicated (Ι² =0%).