Ovarian hormones and reproductive risk factors for breast cancer in premenopausal women: the Norwegian EBBA-I study

results: Mean age was 30.7 years, and age of menarche 13.1 years. Parous women had on average 1.9 births, and age at first birth was 24.5 years. No association was observed between parity and ovarian steroids. In nulliparous women, higher waist circumference (≥77.75 cm) and longer oral contraceptive (OC) use (≥3 years) were associated with higher levels of 17b-estradiol. Short (,10 years) versus long (.13.5 years) ‘menarche-to-first birth’ interval was associated with higher overall mean (Ptrend ¼ 0.029), 47% higher maximum peak and 30% higher mid-cycle levels of 17b-estradiol. We observed a 2.6% decrease in overall mean salivary 17b-estradiol with each 1-year increase in the interval.