Background: Reports on the risk and prognosis of breast cancer in relation to the sex of a child have been conflicting. Since medical sciences play an important role in informing sociocultural understandings of health and illness, evidence-based studies have the potential to foster or counter stigma and shape social attitudes toward a newborn’s sex. Aims: To pool all available evidence to provide the highest level of evidence on the association between the sex of the first child and breast cancer risk or prognosis. Study Design: Systematic review and meta-analyses. Methods: A comprehensive search using three databases was conducted from inception until May 2020. Titles and abstracts of all papers identified were independently screened by two authors. Data extraction and quality assessment were also performed independently by two researchers. The breast cancer risk was quantified using the odds ratio, and the prognosis (i.e., mortality) was measured using the risk ratio. Results: In the meta-analysis, 11 studies with more than 1 million participants were included. The pooled estimate from the five studies on risk and the six studies on prognosis were odds ratio 0.99 (95% confidence interval, 0.95-1.03) and risk ratio 1.00 (95% confidence interval, 0.80-1.26), respectively. Conclusion: When we pooled all available evidence, the sex of the firstborn child was neither associated with risk nor prognosis in breast cancer. Clinically, our findings are reassuring and important, especially in light of previous studies that recommended differential treatment and counseling based on the sex of the first child. Socially, our findings challenge conventional social stereotypes that regard male children as biologically superior to female children.