Number of parity and the risk of non-Hodgkin lymphomas: a dose-response meta-analysis of observational studies.

BACKGROUND: Epidemiological reports have shown that parity is associated with a risk of developing non-Hodgkin lymphomas (NHL). However, the findings have been inconsistent.

METHODS: We searched the EMBASE and PubMed databases for eligible studies up to 10 March 2016. Category and generalized least square regression models were used to perform data analyses.

RESULTS: In total, five cohort and seven case-control studies were identified. Categorical analyses indicated that parity number has little association with NHL and its subtypes. In dose-risk analyses, there were no relationships between parity and NHL risk (pfor association  = 0.064; n = 10). The summarized risk ratio (RR) was 0.97 (95% confidence interval (CI): 0.95-1.00; I2  = 57.8%; pheterogeneity  = 0.014; Power = 0.79) for each additional live birth increase. Similarly, for B-cell NHL, there was a null association between parity and NHL risk (pfor association  = 0.121; n = 5). The combined RR was 0.96 (95% CI = 0.90-1.03; I2  = 63.7%; pheterogeneity  = 0.026; Power = 0.71) for each additional live birth. For follicular NHL, there was still a non-significant association identified (pfor association  = 0.071; n = 4), the pooled RR was 1.00 (95% CI = 0.95-1.07; I2  = 17.3%; pheterogeneity  = 0.305; Power = 0.26) per additional live birth.

CONCLUSIONS: Our data identified little evidence suggesting that high parity is a protective factor against the development of NHL, including its B-cell and follicular subtypes.