After adjusting for age, year at BC diagnosis, stage, positive nodes, and hormone therapy, patients with BC who became pregnant after their cancer diagnosis had lower total mortality than did the comparison group (HR = 0.44, 95% CI = 0.23–0.84), including that of estrogen receptor‐positive patients (HR = 0.23, 95% CI = 0.07–0.77). This evidence concerns the gene ESR1 and breast cancer.