Overweight women may need more frequent mammograms, new research suggests. Women with a higher body mass index can be at a heightened risk of not having their breast tumor detected until it has grown larger.

The study by Swedish researchers was meant to establish risk factors associated with tumors going undetected until they were at least three-quarters of an inch, about the size of a peanut. In a study of more than 2,000 breast cancer cases, the researchers tracked disease progression and its relationship to body mass index and breast density over seven years.

The three-quarter-inch tumor size is something of a tipping point. It is one characteristic that separates stage 1 and stage 2 breast cancer, and tumor size is often associated with prognosis.

For cancers that appeared within two years of a normal mammogram, the patient’s body mass index was found to be a contributing factor. That is, women who were overweight were more likely to have a tumor go undetected between mammograms than women who weighed less. For cancers that were first detected between normal screenings, women with a higher body mass index also had a worse prognosis than women with a lower body mass index, the researchers noted. The findings were presented to the Radiological Society of North America.

The study defined overweight women as having a body mass index of 25 or higher.

Body mass index should be part of the discussion when a physician and patient are discussing the best screening approaches for breast cancer. The researchers also suggested women with a high body mass index get mammograms more often.