By Denise Grady

Centers that provide mammograms to screen for breast cancer will have to tell women whether they have dense breast tissue, which can increase the risk of cancer and mask tumors, the Food and Drug Administration announced in a proposed rule on Wednesday.

Dense tissue can hide cancer from X-rays, making mammography less reliable. Women with dense tissue are often advised to have other screening tests in addition to mammograms, such as ultrasound or M.R.I. scans.

More than half of women over 40 in the United States have dense breasts, the agency said in a statement.

Dense breasts have a high proportion of glandular or connective tissue, which blocks X-rays. Breasts that are not dense have more fat, which X-rays penetrate easily. The only way to detect dense tissue is with a mammogram; it cannot be felt.

Dense tissue makes cancer harder to find on mammograms because the tissue and tumors both show up as white and blend together. Fat looks black, so tumors stand out more in fatty breasts.

About three dozen states already require that women be given information about breast density. The new F.D.A. rules propose specific language that would be used nationwide to explain breast density, note that some women may need more imaging tests and recommend that patients talk to their doctors about their results.

The F.D.A. language would be the minimum required, but states would be free to require more information, Dr. Jeff Shuren, director of the agency’s Center for Devices and Radiological Health, said in a news briefing.

Some in the medical profession have objected to this type of notification, arguing that not all women with dense tissue have the same increased risk of breast cancer, and that reporting the condition could frighten women and lead to a flood of unnecessary screening tests and biopsies. Doctors have also said that many of the state-mandated letters were too hard for patients to understand.

The F.D.A. commissioner, Dr. Scott Gottlieb, who will be leaving the agency on April 5, said in an interview that the new proposal had been high on the list of actions he hoped to take before departing. He acknowledged that some physicians opposed notifying women about breast density, but he said women had a right to receive the information and decide what to do about it.

The state laws mandating that women be told about breast density resulted from the tireless campaign of one woman with breast cancer, Nancy Cappello, a special-education teacher and administrator in Connecticut.

In 2003, just six weeks after being told her mammogram was normal, she learned that she had an advanced breast cancer. When she asked how the mammogram could have missed her large tumor, her doctor told her that she had dense breast tissue. She had never been told that.

She needed a mastectomy, chemotherapy and radiation — a difficult course of treatment that she believed she might have avoided had she only been told about her tissue density so that she could have had additional tests and found the cancer earlier.

Dr. Cappello and her husband took their concerns to legislators, and in 2009 Connecticut became the first state to require that breast density be reported to women.

Dr. Cappello died in November at 66, from complications of a rare blood cancer caused by the chemotherapy and radiation she had received for breast cancer.

Wednesday’s F.D.A. announcement included other proposed changes to mammography regulations, including a provision to allow the agency to notify women and their doctors if a mammogram center failed to meet quality standards and the patient’s test should be re-evaluated or done over at another facility. Other changes tightened rules that govern record-keeping and the equipment used in mammography.

These rules are the first changes proposed in 20 years to the F.D.A.’s regulations on mammography. They will not take effect immediately. They are open for public comment for 90 days and could be modified before becoming final.