Effect of parity and age at delivery on breast cancer risk in Slovenian women aged 25-54 years.

In 1988, a case-control study on breast cancer and oral contraceptives with 624 cases and 624 matched controls in the age range 25-54 years was undertaken in Slovenia. This analysis assesses the relationship between parity and breast cancer risk: the relative importance of age at first birth, age at subsequent births and total parity. We also evaluate whether a dual effect of an increased risk immediately after childbirth followed by a long-term benefit exists. Three logistic regression models were used. Age at first delivery is an important breast cancer risk factor: among parous women it was associated with a 5.3% increase/year in the odds of breast cancer. Multiparity was not shown to be an independent risk factor. Age at subsequent deliveries was associated with a 1% increase in risk for every 1 year increase of age at any birth, but this contribution to the risk was not significant. In the analysis stratified by parity the most important influence is with the age at first birth. We find no evidence of an effect on the odds of breast cancer associated with the age at the second, or later, births. We do find that there is an increased risk associated with the birth of the first child followed by a longer term protective effect. A post-menopausal woman has a reduced breast cancer risk compared with a pre-menopausal woman of the same age, adjusting for the same number of deliveries and ages at these deliveries.