The univariable analysis revealed limited evidence of an effect of age at first birth on HER2-enriched breast cancer risk (UVMR OR: 0.78; 0.47, 1.30), while adjusting for number of births (potential mediator) (MVMR estimated using Q-statistic minimization OR: 0.28; 0.11, 0.57) and ever parous status (genetically correlated factor) (MVMR estimated using Q-statistic minimization OR: 0.38; 0.17,0.80) revealed inverse effects (Additional File 2: Table 4, Additional File 3: Fig. S1). Here, ERBB2 is linked to breast cancer.