NEW YORK (CBSNewYork) — Mammograms save lives, but how often should a woman have one?

New research suggests the answer may depend on your weight.

As CBS2’s Dr. Max Gomez reported, virtually every major medical group in the United States says women should have yearly mammograms after age 40. But a government agency says every two years is enough.

A study now shows if overweight women follow those guidelines, their breast cancers may be found at larger, later stages.

It was almost four years ago that a screening mammogram found a breast cancer in Inez West. It had been about two years since her previous mammogram.

“About two days later I got a note saying ‘come in’,” she said. “I had started losing weight, so I suspected something since my grandmother had breast cancer.”

Perhaps just as important as the delay between mammograms was that Inez had been about 50 pounds heavier around the time of her previous test. Extra weight means extra fat cells, which means increased risk.

“They have chemicals in them that make the body continue to produce more and more estrogen,” Dr. Laurie Margolies from Mount Sinai Chelsea said. “Estrogen is food for the majority of breast cancers.”

Dr. Margolies is the Chief of Breast Cancer Imaging for Mount Sinai Health System. She says a study just presented at the Radiological Society of North America found that if overweight, or high body mass index, women in Sweden waited two years between mammograms, their breast cancers were found to be larger and later stage.

As a result, they were harder to beat.

“People who were thin and screened every year had the greatest chance of having small, early detected breast cancers,” Dr. Margolies said.

So what does that mean for American women? For the Swedish study, high BMI was defined as 25 and above — a category 70 percent of American women fall into. Dr. Margolies said the lesson is clear.

“Screen with a mammography every year, beginning at age 40 and continuing every year as long as we’re healthy,” she said.

Inez says it’s something she’s now doing.

The reason the U.S. Preventative Services Task Force gives for recommending mammography every two years is that yearly mammograms cause undue anxiety in women, but most women say they’d rather deal with some anxiety than a larger, later stage breast cancer.

Studies also show the radiation from modern mammogram machines doesn’t increase cancer risk.