Twelve case-control studies and one cohort study were included; the total number of patients was not stated.
The common ORs for current use of oral contraception were 0.19 (95% confidence interval, CI: 0.15, 0.24) and 1.8 (95% CI: 0.9, 3.4) when compared to non-pregnant and pregnant controls, respectively.
The OR for past use of oral contraception was reported as 1.04 (95% CI: 0.52, 2.1) in a cohort study and 1.2 (95% CI: 0.92, 1.5) in case-control studies.
Results from studies of current use of IUCD show significant heterogeneity. The OR for current use of IUCD were significantly less than 1.0 in 3 of 4 case-control studies when compared to non-pregnant controls, and significantly greater than 1.0 in all 4 studies when compared to pregnant controls. The overall OR was 1.6 (95% CI: 1.4, 1.8) according to 4 case-control studies on past IUCD use.
The common ORs for tubal sterilisation were 0.48 (95% CI: 0.40, 0.59) and 9.3 (95% CI: 4.9, 18) when compared to non-pregnant and pregnant controls, respectively.
The ORs for condom use in a case-control study were 0.18 (95% CI: 0.11, 0.29) and 0.97 (95% CI: 0.24, 4.0) when compared to non-pregnant and pregnant controls, respectively.