Now a new study led by Oxford researchers shows that these factors are particularly relevant for specific types of breast cancer.
The findings reveal more about the way female sex hormones are linked to risk of breast cancer.
Researchers from the University of Oxford found that increases in the risk of breast cancer associated with both earlier periods and later menopause were greater for lobular than for ductal cancers. The increased risk associated with a later menopause was also more marked for oestrogen-sensitive (ER-positive) cancers.
In the most comprehensive study of its kind, the researchers reanalysed original data from 117 studies worldwide, including almost 120,000 women with breast cancer and over 300,000 women without the disease.
The findings are published in The Lancet Oncology. The researchers also discovered that starting periods earlier had a greater impact on breast cancer risk than did finishing periods later – suggesting that the effects of these factors may not simply reflect the number of reproductive cycles in a woman’s lifetime.
‘Our new research should help us towards a better understanding of the way in which female sex hormones affect breast cancer risk,’ said Dr Gillian Reeves of the Cancer Epidemiology Unit at the University of Oxford, one of the lead researchers on the study.
‘We already knew that hormones associated with reproduction have a big impact on breast cancer risk, and that starting periods early and having a late menopause increase risk. But these findings suggest that sex hormone levels may be more relevant for specific types of breast cancer – ER positive tumours, and lobular, rather than ductal tumours.’
The average age in the UK for women to start their period is around 12 years old and most women start the menopause between the ages of 45 and 54.
Hazel Nunn, head of health information at Cancer Research UK, which funded the study, said: ‘Even though women can’t control the age they start their period, there’s many things they can do to reduce their breast cancer risk. Keeping a healthy weight, cutting back on alcohol and having a more active lifestyle will all help to prevent the disease developing.’