Breast cancer risk among women with long-standing lactation and reproductive parameters at low risk level: a case–control study in Northern Tanzania

Breast cancer is the leading cause of death among women worldwide. Studies in industrialised countries identified age at menarche, age at first full-term pregnancy, and lactation as determining factors in the aetiology of breast cancer. 115 female breast cancer patients (cases) and 230 age- and district-matched women clinically free from breast cancer (controls) were interviewed about their reproductive history and socioeconomic condition. Semi-structured interviews including anthropometric measurements were conducted by trained enumerators. The median age was 50 years (min/max 26 to 85 years). Estimated median BMI at age 20 was 21 kg/m² in both cases and controls. Median lifelong lactation of the mothers was 96 months (cases) and 108 months (controls). A high BMI at 20 years was associated with an increased breast cancer risk (OR 1.31 95% CI 1.11–1.55, P < 0.01). The odds ratio for lifelong lactation was slightly below one (OR 0.99 95% CI 0.98–1.00, P < 0.01). There was no significant association in risk for BMI at interview (median 25 kg/m² of cases and 26 kg/m² of controls), age at menarche (median 16 years), and age at first full-term pregnancy (median 20 years). The association of increased risk with higher BMI at age 20 years remained significant after stratification for menopause (premenopausal: OR 1.41 95% CI 1.10–1.81, P = 0.01; postmenopausal: OR 1.38 95% CI 1.06–1.80, P = 0.02). Late age at menarche and prolonged lifelong lactation were associated with a risk reduction among premenopausal women (OR_menarche 0.74 95% CI 0.56–1.00, P = 0.05; OR_lactation 0.98 95% CI 0.97–0.99, P < 0.01). In conclusion, long-standing lactation and reproductive behaviour are associated with a lower breast cancer risk in the region. As current changes in lifestyle affect age at menarche, reproductive behaviour, and nutritional status, an increased incidence of breast cancer is to be expected. Preventive efforts should include advice on reproductive and breastfeeding behaviour.