A possible mechanism was that the mammary cells were repeatedly exposed to high amounts of estrogens and other steroids in closely occurring births, which may stimulate growth and promotion of occult tumor cells.36 Moreover, this effect was expected to specifically influence ER‐positive tumors,6 in line with our result (Table S2) that a long interval between first and second birth was associated with a stronger effect on ER+ breast cancer [HR (95% CI): 0.41 (0.22‐0.75)] than ER‐ counterparts [HR (95% CI): 1.02 (0.55, 1.88)]. This evidence concerns the gene ESR1 and neoplasm.