The authors analyze the heterogeneity present in the combined results of past observational studies that investigated the association between oral contraceptive use and rheumatoid arthritis.

The authors also evaluate discrepancies among meta-analyses that focus on the same relation.

Of the 15 initially reviewed studies, 10 were selected for this meta-analysis, which also includes a qualitative summary of study characteristics and a critical appraisal of study quality.

The authors used the direct method to combine the study results when there was no evidence of heterogeneity and the DerSimonian-Laird method when heterogeneity was present.

Using a meta-regression to assess the sources of heterogeneity, the authors weighted summary estimates by sample size and undertook a sensitivity analysis.

There was a strong indication of heterogeneity when combining all studies (khi2=29.34, p=0.00060) with the source of controls explaining most of the heterogeneity.

The most important factor in explaining the differences among the overall summary estimates given by the meta-analyses is that different effect estimates had been selected for the same studies.

There is no conclusive evidence of a protective effect of oral contraceptives on the risk of developing rheumatoid arthritis.

Consensus is needed on how meta-analyses of observational studies should be conducted.