WASHINGTON (AP) — Deciding when to get routine mammograms is confusing. Some health groups recommend women begin at age 40 or 45 while another recently opted for age 50. They also differ on whether yearly or every other year is best.

The conflicting advice is at least partly because guidelines for breast cancer screening are designed for women at average risk and with no possible cancer symptoms. But breast cancer is so common that it is hard to know who is really “average” and how to balance the pros and cons of screening.

“Breast cancer is not one disease,” said Dr. Laura Esserman of the University of California, San Francisco. “So how in the world does it make sense to screen everybody the same when everyone doesn’t have the same risk?”

Esserman is leading research to better understand the nuances of who is at low or high risk or somewhere in between and eventually offer more tailored screening advice.

More than 320,000 women in the U.S. will be diagnosed with breast cancer this year, according to the American Cancer Society. Death rates have been dropping for decades, thanks largely to better treatments. But it is still the second-most common cause of cancer death in U.S. women — and diagnoses are inching up.

For now, here are some things to know.

When to get a mammogram

The newest guidance comes from the American College of Physicians, which recommends that average-risk women ages 50 to 74 get an every-other-year mammogram. For those 40 to 49, the guideline says to discuss pros and cons with a doctor and if they choose screening, to go every other year.

That advice, issued last month, was a surprise. Most other U.S. health groups have urged women to start earlier, in their 40s. The influential U.S. Preventive Services Task Force recently switched its guidance to start every-other-year mammograms at age 40 instead of 50.

The American Cancer Society has long recommended yearly mammograms for 45- to 54-year-olds — but says they can choose to start at 40. For those age 55 and older, the cancer society says women can switch to every other year or choose to keep going for yearly checks.

The new American College of Physicians guidelines also say doctors can ask if women 75 or older wish to stop routine screening. In contrast, the cancer society says there is no reason to stop if they are still healthy.

Why don’t experts agree?

The higher a woman’s risk of eventually developing breast cancer, the more benefit she will derive from more frequent screenings. But beyond some well-known factors like the cancer-causing BRCA1 or BRCA2 genes, it is hard for women to know their true risk. Age has long been a proxy because the risk of breast cancer rises as women get older.

Mammograms aren’t perfect. Sometimes they miss cancer or an aggressive tumor pops up after a routine mammogram. But guidelines seek to balance the benefits of catching cancer early with possible harms, such as stress and pain from investigating suspicious spots that don’t turn out to be cancerous.