Dense tissue shows up as white areas on a mammogram
Changes in women's breasts are making it harder for doctors to spot cancer, according to experts.
A study in Holland has found that many women in their 50s have breast tissue akin to that of much younger women.
Their breasts contain a much higher proportion of dense tissue, which is difficult to read using mammograms.
Speaking at a European conference on breast cancer, doctors said the changes may be linked to the fact that women are having fewer children.
Dr Fred van der Horst and colleagues at the National Training and Expert Centre for Breast Cancer Screening in Nijmegen studied mammograms from 2,000 women.
They found that 25% of those between the ages of 50 and 69 had high levels of dense breast tissue, which is normally only seen in much younger women. Among 50 to 54-year-olds, the figure was 44%.
Dense tissue is difficult to read on mammograms because it shows up as white areas.
The fear is that if more older women have dense breast tissue, possible cancers may be going undetected.
It is a particular problem for older women because they are most likely to develop breast cancer.
The researchers said ultrasound could be a better way of testing for breast cancer, because it would show up cancerous tissue as black.
However, they said further research is needed before they could recommended such a move.
Dr van der Horst said further research is needed to find out why women's breasts were staying younger for longer.
He suggested hormone replacement therapy was unlikely to be responsible since few of those involved in the study had taken HRT. However, he said changes to family sizes may be a factor.
"We know that women who have given birth have more lucent (less dense) breasts than childless women," he said.
"It's possible that demographic changes such as women having fewer children than 30 years ago and having them at a later age may play a role."
Delyth Morgan, chief executive of Breakthrough Breast Cancer, said the study highlighted the need for better tests for breast cancer.
"We know that mammographic screening is less effective at detecting cancer in denser breast tissue, and this is a particular problem for younger women, needing regular screening because of their family history of the disease.
"More research is needed to develop better ways of detecting breast cancer earlier and more accurately in denser breasts and this research only compounds that need."