Number of parity and the risk of rheumatoid arthritis in women: A dose-response meta-analysis of observational studies.

AIM: The association between parity and rheumatoid arthritis (RA) risk has been investigated, but results are controversial. Thus, our aim was to systematically analyze the effect of number of parity on the risk of RA in women.

METHODS: Relevant published studies were identified using PubMed and embase databases through 1 April 2016. We pooled the relative risks (RR) and 95% confidence intervals (CI) using random-effects models.

RESULTS: In all, 12 studies with a total of 2 497 580 participants and 11 521 RA cases were included. A borderline significant inverse association was observed when we compared parity with nulliparity for RA, with summarized RR = 0.90 (95%CI: 0.79-1.02; I2  = 58.5%, Pheterogeneity  = 0.010). In dose-response analysis, we observed a significant nonlinear (Pnonlinearity  = 0.000) relation between parity number and the risk of RA. Compared with null parity, the pooled RR of RA were 0.89 (95%CI: 0.86-0.93), 0.84 (95%CI: 0.79-0.89), 0.85 (95%CI: 0.79-0.90), 0.88 (95%CI: 0.81-0.95), 0.90 (95%CI: 0.83-0.97), 0.92 (95%CI: 0.84-1.02), and 0.94 (95%CI: 0.83-1.07) for 1, 2, 3, 4, 5, 6, and 7 live births, respectively. Subgroup and sensitivity analyses showed similar associations. No publication bias was found.

CONCLUSION: The findings from the current meta-analysis indicate that parity was related to decreased risk of RA. The greatest risk reduction appeared when the parity number reached two. Further studies are warranted to confirm our findings.