Left-handed women are more than twice as likely as right-handers to suffer from Breast cancer before reaching Menopause, Dutch scientists said on Monday.

More than a million women are diagnosed with Breast cancer worldwide each year. Three-quarters of cases occur after Menopause, which usually begins around the age of 50.

Researchers at the University Medical Center in Utrecht in the Netherlands speculate that there is a shared origin early in life for both left-handedness and developing Breast cancer, possibly exposure to hormones in the womb.

“Left-handedness is associated with Breast cancer, most specifically pre-menopausal Breast cancer,” said Cuno Uiterwaal, an assistant professor of clinical epidemiology at the university, in an interview.

He and his colleagues studied 12,000 healthy, middle-aged women born between 1932-1941 who were part of a breast screening program. The scientists determined their hand preference and followed up their medical history to see which women developed Breast cancer.

“If we take pre-menopausal and post-menopausal Breast cancer then there was a 40 percent increased risk,” Uiterwaal said of left-handed women. But when they investigated further the scientists found most of the excess risk was in Breast cancer before the Menopause.

“We found that left-handed women are more than twice as likely to develop pre-menopausal Breast cancer as non-left handed women,” the researchers said in the report published online by the British Medical Journal.

Other risk factors such as family history of Breast cancer, numbers of pregnancies, smoking habits, and social and economic status were considered.

About 8 percent to 9 percent of women are left-handed. But the scientists said the findings should not alarm them.

“What our study intends to do is focus on this area. We do not know all the causes of Breast cancer, that is why we should continue. This may be one new factor that leads us to a better understanding of the aetiology (cause of the illness),” Uiterwaal added.

About 5 percent to 10 percent of Breast cancers are hereditary. Most are due to mutations in the BRCA1 or BRCA2 genes. The earlier the illness is diagnosed and treated, the better the prognosis is for the woman.

“Although the underlying mechanisms remain elusive, our results support the hypothesis that left handedness is related to increased risk of Breast cancer,” the researchers added.