SANTA BARBARA, Calif. -

A new law is in effect that will have an impact on up to 50 percent of women who get mammograms in California.

The law that began as Senate Bill 1538 will now make it mandatory for doctors to notify women if they have dense breast tissue.

The National Cancer Institute recommends women age 40 and older get a mammogram every one to two years. But that routine check might not cover everyone.

“Almost always when you hear someone has had a breast cancer diagnosis and they say, ‘And just two months ago I had a normal mammogram,’ it’s a woman with dense breast tissue,” said Dr. Judy Dean, the radiologist at the Santa Barbara Women’s Imaging Center.

On a mammogram, dense breast tissue shows up white, which is the same color as a tumor. That makes it difficult to tell the difference.

“It’s been described as trying to see a snowball in a blizzard,” said Dean.

Forty to 50 percent of women have some degree of dense breast tissue and may not know it. Now, they will be notified when they get their mammogram results if they have the kind of tissue that sometimes covers up cancer.

Dean testified at the California State Senate on why the notification is so important.

She is glad that it’s now the law for doctors to share that information with their patients so they can get further testing.

Some of the other options include 3-D mammography, which shows tissue in overlapping layers, and ultrasounds, which have proven to be effective even when mammograms are not.

“It’s up to 42 breast cancers that I’ve detected personally with ultrasound that are not evident on their mammograms, and 36 are so small they are not a palpable lump either. They would not have been detected without ultrasound,” Dean said.

With the new law in effect, women will now know if they have dense breast tissue and if they should see a doctor for further screening.