New Jersey is the latest state to require mammography centers to give patients information about the density of their breasts. Pennsylvania passed a similar law last year.

Dense breast tissue can make it hard for a radiologist to interpret the results of a mammogram. More than a dozen states now have a breast-density notification law.

Obstetrician-gynecologist Sherry Blumenthal said the laws aren't fully backed by science, but if lawmakers oppose the legislation they are perceived as "anti-woman."

"The problem is from a political point of view," said Blumenthal, a member of the board of the Pennsylvania Medical Society.

"It was a conundrum, and this is why it has passed everywhere, even if the legislators don't understand the purpose of it or the medical issues, they can't vote against it," she said.

Several medical organizations have expressed concerns that the laws will cause confusion and unnecessary follow-up testing.

Back in 2012, The American College of Radiology released a statement in the midst of a similar debate, and now has a brochure for patients to learn more.

The ACR "recommends that all stakeholders proceed with caution in considering a statutory or legislative mandate to include breast parenchymal density information in the patient summary or to require that patients receive copies of their imaging reports sent to their ordering physician."

Support for the laws has been strong among some non-medical advocacy groups, Blumenthal said.

"They have been pushing legislation going from state to state," she said.

Nancy Cappello, the founder of Are You Dense Advocacy Incorporated, said the density notification letters prompt women to have a follow-up conversation with a doctor, so they can better understand their personal risk breast cancer.

She said she often hears from women who have dense breast tissue and thought they had the all clear after a mammogram.

"They say, 'I just had a mammogram and it was normal ... and I find six weeks later, two months, or three months later, that I have cancer and it's large, and it's late stage.' So that's a real issue -- the masking effect," Cappello said. "So what we want to happen is for women to find out prior to cancer."

She has a similar personal story and later launched her nonprofit.

Opponents of the notification law say it may put many women on high alert who don't need to be concerned.

"We feel that the legislators should stay our of our exam room," Blumenthal said. "We are the ones with the training, we are the one with the knowledge to take care of patients to discuss these things and make decisions with patients."

Obstetrician-gynecologist Sharon Mass, who practices in Morristown, N.J., and is a leader with the American Congress of Obstetricians and Gynecologists recommends the site www.breastdensity.info for women who want to find out more.