Relationship between parity and breast cancer risk: A critical review of evidence (with focus on sub-Saharan Africa)

The association between parity and breast cancer has been widely investigated in high-income countries. However, in sub-Saharan African (SSA) region, only a few studies have investigated this association. The aim of this study was to critically review the available evidence, highlighting their strength and weakness, thereby providing a basis for more studies in SSA on the topic. A literature search in MEDLINE, Scopus, PubMed, Google Scholar, and reference list of published studies was conducted. Key case control and cohort studies published between 2000 and 2018 were selected for detailed review based on sample size, geographical distributions, and relevance of the findings. Nonanalytical studies were excluded. The findings were summarized using the tables and forest plots based on R Statistical Software version 3.4.0 (2017). This was primarily to provide visual overview of the reviewed studies and not necessarily to provide a pooled estimate. The conclusion was drawn by comparing the number of observed significant findings (assessed based on confidence intervals) with the number of nonsignificant findings across countries and SSA taking into considerations their strengths and weaknesses. Twenty-six case - control studies and 12 cohort studies across the four regions of the world North America, Europe, Asia, and Africa were reviewed. A reduced risk of breast cancer (especially estrogen receptor [ER] positive breast cancer) was observed in most studies, especially studies from high-income countries. While several studies that reported a nonsignificant observation in SSA were limited in sample size, the few that observed a reduced risk of breast with parity among all women were subject to confounding. Nevertheless, recent studies suggest that the role of parity may be modified by ER status and menopausal status. Further investigation of these suggestions will be needed for a clearer understanding of the relationship between parity and breast cancer in SSA.