Preterm Birth and Breast Cancer Risk: A Systematic Review and Meta-Analysis

BACKGROUNDS: Changes in estrogen levels during pregnancy as well as histologic changes in breast tissue can justify the relationship of preterm birth (PTB) and the risk of BC. Therefore, there is a hypothesis that the duration of pregnancy can be associated with BC, so the aim of this study was to find out whether PTB is a risk factor for BC. METHODS: Published studies were located back to the earliest available publication date (1983), using the Medline/PubMed, Embase, Scopus, and Web of Science bibliographic databases. This review included the cohort or case control studies that assessed the association between PTB and BC. Pooled effect sizes with corresponding 95% confidence intervals (CI) were calculated using random-effects models. RESULTS: Thirteen studies including a total of 2,845,553 women were included in this meta-analysis. Pooled results suggested that PTB could increase the risk of BC (RR: 1.03, 95% CI: 1.00, 1.07; I2= 62.5%). The risk was significantly increased in women who delivered at 37-39 (RR: 1.03, 95% CI: 1.01, 1.06) and 26-31 weeks of gestation (RR: 1.25, 95% CI: 1.04, 1.47) compared to women who delivered at 40-41 weeks of gestation. A significant increment in the risk of BC was observed in primiparous (RR: 1.05, 95% CI: 1.01, 1.08) and women older than 45 years (RR = 1.12, 95% CI: 1.01, 1.24). There was no difference between other gestational age categories. CONCLUSIONS: Our findings add to evidence that short gestation pregnancies may increase the risk of BC, especially in primiparous and women older than 45 years. Considering the methodological weaknesses existed in included studies, minor clinical differences, and the complexity of the exact pathophysiology of PTB on BC, the precise position of PTB as a risk factor for BC in clinical practice is undetermined. Further studies are still needed.