Cancer experts said earlier detection of breast tumours, and better treatments, including chemotherapy and hormonal drugs, appear to be having an impact on the disease. Changes in the age at which women started their families during the post-war years are also a factor.
Analysis of the latest figures by the Imperial Cancer Research Fund's Epidemiology Unit show that overall death rates in women under 80 in 1993 were 10 per cent lower than in1985-1989. In women aged 20 to 49, the mortality rate fell by 14 per cent between 1985-89 and 1993; by 11 per cent in women aged 50 to 69, and 5 per cent in women aged 70 to 79.
Dr Valerie Beral, head of the ICRF unit in Oxford, said: "The fall in death rates has been very rapid in the past few years and the decline is due to a number of reasons. Part of the decline is a consequence of changes in child-bearing patterns in the last decades."
Women who delay starting a family until their thirties run a greater risk of developing breast cancer than women who have children in their early twenties or younger. This is thought to be related to the exposure of breast tissue to oestrogen which is greater in non-pregnant, non-lactating women. Breast-feeding is also known to be a protective factor against the disease.
Between the 1950s and the 1970s the average age at which women had their first child fell from 25-26 during the 1920s to late1930s, to 23. The number of women not having children fell from 20 per cent after the First World War to 10 per cent after the Second World War.
In the past 10 years there has been a reversal of the trend of earlier child-bearing as women have chosen to concentrate on their careers and delay starting a family. And more women are deciding not to have children.
However, Dr Beral said the fall in death rates was too rapid to be wholly due to changing childbirth patterns. "There is an increasing awareness of breast cancer among women and doctors, and women have probably begun to receive treatment earlier. Early treatment saves lives. Also, they are receiving better treatments in the past."
Writing in tomorrow's issue of the Lancet, Dr Beral says the decrease was "too soon and across too wide an age-span" to have been prompted by the NHS breast screening programme.