In a move that has irked medical groups and delighted patient advocates, states have begun passing laws requiring clinics that perform mammograms to tell patients whether they have something that many women have never even heard of: dense breast tissue.

Women who have dense tissue must, under those laws, also be told that it can hide tumors on a mammogram, that it may increase the risk of breast cancer and that they should ask their doctors if they need additional screening tests, like ultrasound or MRI scans.

The issue is pitting angry patients against the medical establishment. Advocates say women have a right to know, but medical groups argue that the significance of tissue density is uncertain and that reporting it may panic women and lead to an avalanche of needless screening tests and biopsies.

Laws requiring disclosure have been passed in Connecticut, Texas and Virginia, and most recently in California and New York, where they will take effect next year. A bill calling for a federal law has been introduced in the House.

The laws owe their existence mostly to Nancy Cappello, 59, of Woodbury, Conn. She was not told that she had dense breast tissue until after doctors found an advanced cancer that mammograms had missed. She took her story to legislators, and in 2009, Connecticut became the first state to require that women be told if they have dense breasts and that insurance companies cover ultrasounds. "I want to help other women," said Cappello, formerly the state's chief of special education.

"Dense" breasts have a relatively high proportion of glandular or connective tissue, which blocks X-rays. Non-dense breasts have more fat, which X-rays penetrate easily. Overall, about 40 percent of women who have mammograms have dense breast tissue. It is not abnormal, just one of nature's variations. Younger women are more likely to have dense tissue, but as many as 25 percent of older women do too. Density cannot be judged by touch; it shows up only on mammograms.

Studies have found that when women with dense breasts were given mammograms and then ultrasounds, the ultrasound found tumors that the mammograms missed -- but also produced many false positives that led to biopsies. Medical groups also argue that identifying dense breast tissue is subjective, and so two doctors reading the same mammogram may rate the tissue differently.