Comparative study of breast cancer risk factors in Estonia and Slovakia. East-European Study Group of Breast Cancer Epidemiology.

Selected more or less known risk factors of breast cancer were investigated and compared in this case-control study in Estonian and Slovak women. The results revealed that women who gave birth to the first child before the age of 20 years had one-fifth and one-third breast cancer risk in Estonia and Slovakia, respectively, in relation to nulliparous women. Concerning subsequent births, Estonian women with two children showed the lowest relative risk, while in Slovak women this risk decreased with increasing parity. Only in women from Slovakia who gave the first birth at, and after the age of 30 years, significantly elevated breast cancer risk was observed. In this study there was no evidence of an association between breast cancer risk and the age at menarche, although an increasing trend of the variable early menarche was apparent in both female populations studied. In addition, an increase in the risk was revealed among obese Estonian women, and thyroid diseases were found to be related with a twofold excess of breast cancer risk in Slovak women. The possible existence of differences in risk factors in the populations showing similar breast cancer incidence rates is discussed.