Backgrounds Reproductive factors, such as age at menarche, parity, age at first birth, and breast feeding, are known to be associated with risk of breast cancer. Recent studies show the association between the reproductive factors and the molecular subtypes of breast cancer, especially in luminal-like subtypes. Nulliparity is known a risk factor of breast cancer, multiparity is considered a protective factor. However, pregnancy associated breast cancers show the high expression of HER2, Therefore, we hypothesized the number of birth (multiparity) is correlated with the over-expression of HER2. Methods We reviewed the database of 2,594 breast cancer patients were over 20 years of age, diagnosed between January 2000 and April 2017 at Korea University of Anam Hospital, Seoul, Korea. According the number of birth, nulliparous (the number of birth = 0; 271 patients), primiparous (the number of birth = 1; 420 patients) and multiparous (the number of birth≥2; 1,903 patients) groups were classified. Results The median age (range) of nulliparous, primiparous and multiparous groups was 43 (22-77), 48 (27-87) and 51 (26-86) years (P<0.001). In univariate analysis, the number of birth was associated with the expression of ER and HER2, nulliparous patients showed higher ER positivity (P=0.013) and multiparous patients showed higher HER2 overexpression (P=0.048). There was no association with PR and Ki-67 level. In logistic regression with age, the parity was negative correlation with ER positivity (OR 0.83, 95% CI 0.72-0.95, P=0.010) and positive correlation with HER2 overexpression (OR1.18, 95% CI 1.02-1.36, P=0.021) P=0.009) and multiparity was higher HER2 overexpression (OR 1.41, 95% CI 1.02-1.94, P=0.033). By subgroup analysis with menopausal status, there was statistically significance in only premenopausal women, not in postmenopausal women. In premenopausal women, the parity was negatively associated with ER positivity (OR 0.78, 95%CI 0.64-0.94, P=0.010) and multiparity was associated with HER2 overexpression (OR 1.72, 95%CI 1.12-2.63, P=0.013), there was no difference in HER2 overexpression between nulliparous and primiparous patients. Conclusion Parity was associated with ER and HER2 expression, multiparous patients were associated with ER negativity and HER2 overexpression, especially in premenopausal women. This suggests that pregnancy could affect risk of breast cancer, especially in HER2 positive breast cancer subtype (ER- HER2+ tumors), before menopause. We need further investigation and evaluation. Citation Format: Bae SY, Kim J, You JY, Jung SP, Bae JW. The association age-related parity and HER2 over-expression in breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-10-09.