Parity and risk of lung cancer in women: systematic review and meta-analysis of epidemiological studies.

Multiple studies have assessed parity as a risk factor for lung cancer but results have been inconclusive. We searched MEDLINE (through August 2010) and the Institute of Scientific Information Web of Knowledge database (through April 2011) to identify studies investigating the association of parity with lung cancer and allowing the calculation of dose-response trends using a linear model. Between-study heterogeneity was assessed using Cochran's Q statistic and the I(2) index. Summary per-child relative risks (RRs) with their 95% confidence interval (CI) were estimated using random effects meta-analysis. Sixteen eligible studies (8077 lung cancer patients; 350,295 unaffected individuals) provided data for meta-analysis. There was significant between-study heterogeneity (p<0.001; I(2)=73%). The summary per livebirth RR was 0.98 (95% CI, 0.95-1.02), indicating no effect of parity on lung cancer risk. Results were consistent in case-control (n=11), RR=0.99 (95% CI, 0.94-1.04), and cohort studies (n=5), RR=0.97 (95% CI, 0.92-1.03). Studies not including small-cell lung cancer patients found a borderline protective effect of parity, RR=0.94 (95% CI, 0.88-1.00). In contrast, no effect was observed in studies including small-cell lung cancer patients, RR=1.00 (95% CI, 0.98-1.03); p for difference=0.05. Overall, there was little evidence of a dose-response relationship between increasing number of livebirths and lung cancer; however, studies have produced heterogeneous results. Future studies should include analyses in well-defined histological disease subgroups.