Parity and breast cancer: evidence of a dual effect.

Epidemiological studies have shown that early first pregnancy reduces the risk of developing breast cancer, which indicates that initiation of the disease occurs at an early age. Thus the subclinical lesion of breast cancer might already be present in the breast before childbearing begins and the growth of any such focus might be modified by the endocrine changes of pregnancy. To test this hypothesis the relation between parity and age at presentation was studied in 341 unselected patients with breast cancer presenting to a single clinic. The mean age at presentation was 5.2 years lower in parous than nulliparous women (p < 0.001) and fell with increasing parity. It is concluded that reproductive history influences not only the risk of breast cancer but also the latent interval of a proportion of breast carcinomas.