Parity and age at first childbirth in relation to the risk of different breast cancer subgroups.

The aim of the present study was to examine parity and age at first childbirth, in relation to the risk of specific breast cancer subgroups. A prospective cohort, The Malmö Diet and Cancer Study, including 17,035 women were followed with linkage to Swedish Cancer Registry until December 31, 2004. A total of 622 incident breast cancers were diagnosed during follow-up and were evaluated regarding invasiveness, tumour size, axillary lymph node status, Nottingham grade, tumour proliferation (Ki67), HER2, cyclin D1 and p27. The tumours were also examined for WHO type and hormone receptor status. Nulliparity was associated with an overall increased risk of breast cancer, although not statistically significant (the relative risk was 1.39 with a 95% confidence interval of 0.92-2.08). Nulliparity was also associated with large tumours (>20 mm) (1.89: 0.91-3.91), high Ki67 levels (1.95: 0.93-4.10), high cyclin D1 levels (2.15: 0.88-5.27), grade III (2.93: 1.29-6.64) and HER2 positive tumours (3.24: 1.02-10.25). High parity was not statistically significantly associated with any specific breast cancer subgroup. Older age at first childbirth (>30) was associated with a slightly increased risk of breast cancer (1.39: 0.94-2.07). There was a statistically significant association between late first childbirth and lobular type (2.51: 1.01-6.28), grade III tumours (2.67: 1.19-6.02), high levels of cyclin D1 (2.69: 1.18-6.12) and low levels of p27 (2.23: 1.15-4.35). We conclude that nulliparity and late first childbirth are associated with relatively more aggressive breast cancer subgroups.