The team studied over 2, 500 women involved in hormone replacement therapy - whose average age was 66.

BBC: Annual post-menopause smears 'unnecessary'

Prolonged oestrogen secretion would certainly keep a woman fertile longer, and it might also keep her healthier in old age as those who use hormone-replacement therapy are well aware.

ECONOMIST: Ageing

But another explanation could be changes in lifestyle factors which increase the risk of hormone-dependent tumours, such as women having babies at an older age, obesity after menopause and use of hormone replacement therapy.

BBC: Breast cancer biology 'changing'