Though such testing is standard in the US for that age group with a family history, women in Europe who have an intermediate family risk are not always offered screening if they are under 50.
British researchers followed 6,710 women under 50 who had a moderate breast cancer risk - meaning women who meet criteria such as having one close female relative who had breast cancer in her 40s or younger. Such women have double to triple the risk of having breast cancer as the average woman.
The women were monitored for about five years and given a yearly mammogram for about four years. A total of 136 women from the group were diagnosed with breast cancer during the course of the study.
The mammogram study
Doctors thought it would be unethical to have a comparison group that was not offered screening. So they used two previous studies: of women who were not screened and of breast cancer patients with a family history.
The study was too short to show whether the screening saved lives, but using tumour size and severity, Duffy and colleagues estimated annual mammograms reduced the death rate by 20% in women with a moderate family breast cancer risk.
The tumours they found were also smaller and less likely to have spread when compared to women who hadn't been screened and later had cancer. The scientists calculated two women would be saved for every 1,000 women screened over a decade.
The research was published in the medical journal Lancet Oncology. It was paid for by the UK National Health Service Technology Assessment.
Mammograms in Europe
In Britain and elsewhere in Europe, women can get mammograms every two years from age 50 to 70 unless they have a high risk of breast cancer, in which case they may be tested earlier.
"At the moment, whether or not women with a moderate family risk get tested varies considerably," said Stephen Duffy, professor of cancer screening at Queen Mary University of London and one of the study authors.
"We hope our findings will push governments to make this kind of mammography universal for these women."
Policies and opinions vary on whether women at average risk should be screened before 50. A US health task force last year concluded that women at average risk for breast cancer don't need mammograms in their 40s but not all groups agreed with that decision.
"All women over 40, both those at normal risk and intermediate and high risk, should be getting mammograms on a regular basis," said Otis Brawley, chief medical officer of the American Cancer Society.
Karsten Jorgensen of the Nordic Cochrane Centre in Copenhagen said doctors are over-treating breast cancer and many women who are screened will test positive for a cancer that would never have threatened their health.
"Any woman needs to consider the harms and benefits carefully before she decides if she wants to be screened," he said.
(Sapa, Maria Cheng, November 2010)