In most states, when a woman gets the results of a mammogram, she also by law receives information about breast density. That’s the degree to which a woman’s breasts are made up of ducts, glands and connective tissue versus fatty tissue. And density matters — it’s a risk factor for breast cancer and it makes cancer harder to detect. In 2009, Connecticut became the first state to pass a law requiring that women who have mammograms are told about breast density — a characteristic that can only be seen on a mammogram — and today, 36 states have similar laws.

It’s clear that mammograms are a less effective screening tool for women with dense breasts, but what’s not clear is what women with dense breasts should do. The U.S. Preventive Services Task Force’s current recommendations say there’s not enough information to know whether offering these women additional screening will help them, and so many women are being given potentially concerning news without any proven ways to address it. The legislators, in other words, have gotten ahead of the scientists, who are still grappling with how to advise women with dense breasts.

Density is a spectrum, not a binary measure. Radiologists have classified it into four categories, with almost entirely fatty breasts on one end, and extremely dense breasts at the other. When physicians talk about “dense breasts,” they mean the top two categories, which include about 43 percent of U.S. women between the ages of 40 and 74. That makes it a very common risk factor, affecting tens of millions of women. To complicate things, density classifications depend on a radiologist’s reading and judgment calls, so they’re not necessarily consistent over time or between practitioners. And density can change over a woman’s life span; dense breasts are more common among younger women, for example.

A study published in the Journal of Clinical Oncology in 2010 found that those top two density categories were most strongly associated with cancer among premenopausal women and postmenopausal women using hormone replacement therapy.

Five-year risk of a breast cancer diagnosis in different groups of women, by breast density Pre-menopausal women ages 45-49 Breast density five-year risk Low 0.7% Average 1.3 High 2.1 Very high 2.5 Postmenopausal women ages 50-54, without hormone replacement Breast density five-year risk Low 0.6% Average 1.3 High 1.8 Very high 2.2 Postmenopausal women ages 50-54, with hormone therapy breast density five-year risk Low 0.6% Average 1.3 High 2.3 Very high 2.8 Source: Journal of Clinical Oncology

Scientists have known for many years that breast density is a risk factor for breast cancer without knowing exactly why. Women, however, weren’t necessarily part of the conversation. In 2004, Nancy Cappello was diagnosed with stage 3C breast cancer that had spread to her lymph nodes, despite having a normal mammogram just six weeks earlier. She found out she had dense breasts, something she’d never been told about before.

Cappello formed a nonprofit called Are You Dense?, which has advocated for the breast density notification laws that now exist in most states. They vary in their language and scope — some states require only women in the top two categories to be notified about their density; some only mention the potential for density to mask tumors, not cause more of them; and some require only general information about risk without telling a woman into which category she falls. But the bottom-line message is that women should talk to their doctors about their density. Cappello was lead author on a recent survey, published in the Journal of the American College of Radiology, which shows that women overwhelmingly want this information.

But then what do they do with it? “The hard thing about breast density is there’s not a lot of action you can take to change that risk factor,” said Christine Gunn, a research assistant professor at the Boston University School of Medicine whose research interests include risk communication and decision-making. At this point, most of the discussion has focused on offering women with dense breasts additional screening with imaging tests like ultrasounds, 3D mammograms and MRIs. But there is no consensus on what type of extra screening is the best — or whether any of them will reduce breast cancer deaths.

It’s important to note that breast density is associated with a greater risk of being diagnosed with cancer, but it’s not associated with a higher risk of dying from breast cancer — and preventing deaths from a disease has traditionally been the gold standard for a screening test. Mammography hasn’t done much to reduce the incidence of metastatic breast cancer, and how often women should get mammograms remains a point of contention. Cappello said that there’s a benefit to finding breast cancers at an earlier stage, since advanced cancers have worse survival outcomes and require more harsh and invasive treatments. But there are potential downsides of screening as well: anxiety, false positive findings — possibly with unnecessary biopsies — and overdiagnosis in which tumors that never would have posed a threat to a person’s health are detected and treated. “The more you look, the more you find,” said Joann Elmore, professor of medicine at David Geffen School of Medicine at UCLA. And with our current technology and understanding of cancer biology, we can’t tell women that one tumor should be treated and another shouldn’t, said Elmore. So — quite understandably — they are all treated.

Whether or not women with dense breasts should get extra screening depends on who you ask and how they weigh the potential benefits and risks:

The U.S. Preventive Services Task Force’s current recommendations say there’s not enough information to gauge the balance of benefits and harms of extra screening (with any method) for women with dense breasts and negative mammograms.

The American Cancer Society says there is not enough information to say whether women with dense breasts should get an extra screening with MRI unless they are already in a high-risk group.

The American College of Obstetricians and Gynecologists doesn’t recommend supplementary testing in women with dense breasts who are asymptomatic and have no other risk factors.

The American College of Radiology said this year that women with a personal history of breast cancer and dense breasts should be screened with MRI , and women with dense breasts but no other risk factors can consider additional screening with ultrasound after discussing the risks and benefits with their doctors.

“Everyone agrees that if you screen more, you will find additional cancers,” said Christoph Lee, a professor of radiology at the University of Washington School of Medicine. “But with the additional screening comes benign biopsies and false positives.” Moreover, insurance coverage isn’t consistent, which means women may pay out of pocket for extra screening that may or may not help, he said.

Women should know their breast density, said Karla Kerlikowske, a professor of medicine and epidemiology and biostatistics at University of California San Francisco and lead author of that 2010 study on breast cancer risk and density. But that information should be consistent — everyone should know, regardless of the category they fall into. (A woman with low overall risk and fatty breasts may rest easier knowing she is at even lower risk of breast cancer.) “If there’s a moral imperative to give information, there’s a moral imperative to give better information,” said Saurabh Jha, a radiologist who has written extensively about screening.

To really gauge a woman’s cancer risk, density can’t be considered in isolation — it needs to be discussed in the context of other risk factors, such as family history and previous breast biopsies. The Breast Cancer Surveillance Consortium Risk Calculator can estimate a woman’s risk taking all of these factors into account. Breast density is just one part of a woman’s overall risk profile, which is what women really need to know, Kerlikowske said.