40% of women have this. Why you should care.

POSTED: 03:34 PM EDT Oct 31, 2013    UPDATED: 05:49 PM EDT Oct 31, 2013 
ROANOKE, Va. -

Some women are receiving a letter after having a mammogram that says her mammogram is normal but she has dense breast tissue and should talk to her doctor about it. 

It's because of a new law in Virginia that requires doctors to let women with dense breast tissue know about it.  So what does it mean and why should you care?

Dr. Eileen Kenny is the Medical Director of the Carilion Breast Care Center and Section Chief of Breast Imaging at Carilion Clinic in Roanoke.  She showed us what  breast cancer looks like on a mammogram.  It shows up as white.  Dense breast tissue also looks white on a mammogram.

"The problem we have with breasts that are dense is they have a lot of that white tissue and breast cancer is white so it hides in that white tissue," Dr. Kenny said.

About 40 percent of women have dense breast tissue in the United States.  If you have a normal mammogram and find out you have dense breast tissue talk to your doctor about it, Dr. Kenny said.

"Their doctor might want to send them back for breast ultrasound," said Dr. Kenny. "It's another type of test that might look at their breast tissue more fully and pick up extra cancers which ultrasounds can do."

Ultrasounds are not foolproof either. There can be more cases of false positive readings and insurance companies are not yet covering the cost  for preventative screening, according to Dr. Kenny.

The best advice for women with dense breast tissue is to talk with their doctor and then weigh the harms versus the benefits to decide the best course of preventative screening for them,  said Dr. Kenny.