Number of parity and the risk of gallbladder cancer: a systematic review and dose-response meta-analysis of observational studies.

Background: Current epidemiological evidence suggests an association between parity and risk of gallbladder cancer (GBC), but studies have yielded inconsistent conclusions.

Objective: The purpose of this meta-analysis is to systematically analyze the effect of the number of parity on GBC risk.

Methods: We searched Web of Science, EMBASE, PubMed, China Biological Medicine Database from inception to the end of April 2015. Studies investigating parity and risk of GBC were included. A systematic review and a dose-response meta-analysis were performed to investigate the association between parity and GBC risk using odds ratios (OR) and 95 % confidence intervals (CI).

Results: Thirteen case-control studies were eligible for inclusion in this meta-analysis, including a total of 2,164 cases and 11,340 controls. A significant association was identified for the ever parity vs. nulliparous at 1.39 (95 % CI 1.15-1.68; Power = 0.73; I (2) =0.0 %; P = 0.90). Similarly, the summary estimate for high vs. low parity number was 1.86 (95 % CI 1.51-2.30; Power = 0.26; I (2) = 66.0 % P < 0.01). For the dose-response relationship, a non-linear association between the parity number and GBC risk was not observed (P non-linearity = 0.578), but a clear linear relationship was detected. The combined odds ratio of GBC for an increase in parity of one live birth was 1.12 (95 % CI 1.09-1.21; Power = 0.99; I (2) = 39.9 %; P = 0.139). Subgroup and sensitivity analyses showed similar associations. No publication bias was found in all results. Significant heterogeneity between subgroups was detected by meta-regression analyses.

Conclusions: In females, higher parity may be associated with an increased risk of gallbladder cancer. In the future, high-quality cohort studies with larger sample sizes and randomized controlled trials are needed to fully scrutinize this association.