It's a risk factor for breast cancer many women don't even know they have but a new Wisconsin state law will require doctors to tell them.
Doctors say nearly half of all women have dense breast tissue that can hide cancer on a mammogram. It also in itself makes a women more likely to develop breast cancer.
It's something Anne Zellner said she had no idea about until four months after receiving a clean mammogram, she was diagnosed with Stage 3 breast cancer.
"Of course I was in shock as anyone would be," she said. "I thought, 'How could this have happened to me?'"
After going through chemotherapy, radiation and a mastectomy, she joined forces with another Wisconsin woman to pass a law requiring doctors to tell women if they have dense breast tissue.
Gov. Scott Walker signed the bill into law on Tuesday.
"Their chances for treatment and management are much better," said Dr. Georgia Giakoumis Spear, the Chief of Breast Imaging at NorthShore University HealthSystem.
She said they already supplement their patients' mammograms with automated breast ultrasounds.
"Cancer appears white on a mammogram, density appears white on a mammogram," she said. "It's like trying to search for cancer through the clouds."
Now that Wisconsin women will know their density level, Spear says they can ask for additional tests.
"She can advocate for herself and look for additional tools as necessary or discuss this with her doctor," Spear said.
Additional screenings could lead to earlier detection.
"If I can prevent someone else from going through that or help someone else catch their cancer sooner, it would just make all the difference in the world," Zellner said.
Spear said there are signs of cancer that only a mammogram can find. That's why she says it is still the gold standard for breast cancer screenings.
Women are encouraged to start getting mammograms at age 40.
Wisconsin is now the 35th state to pass a law requiring doctors to tell women about dense breast tissue.