Just when you think you understand when to start getting a mammogram and how often to repeat the breast cancer screening test, the recommendations change.

In October, the American Cancer Society (ACS) changed its guidelines, moving the recommended age at which women should start getting mammograms. Their guidelines, as before, are different from those of other organizations, such as the U.S. Preventive Services Task Force.

So whose directives should you follow, and why are they different? Here’s the lowdown.

Related: Do You Need a 3-D Mammogram?

Who recommends what?

Under the new ACS guidelines, women with an average risk of breast cancer (not those with genetic mutations or other factors that put them at increased risk) can start mammograms at age 45 instead of 40. As before, the guidelines advise women to repeat the test annually.

However, they now say women can switch to getting tested every other year once they reach age 55. Women are advised to continue mammograms as long as they are in good health.

Several other organizations offer mammography guidelines, too. The U.S. Preventive Services Task Force (USPSTF), which updated its guidelines in 2009, suggests starting at age 50 and repeating the test every two years. The American College of Obstetricians and Gynecologists recommends annual screening beginning at age 40. The American College of Radiology and the Society of Breast Imaging also recommends annual screening beginning at age 40.

Related: MRIs Might Predict Breast Cancer in Some Women

Why do guidelines differ?

"The task force and the ACS look at the same evidence, but they give different weighting'' to different types of evidence, says Richard Wender, MD, chief cancer control officer for the ACS.

While the ACS give more weight to the results of observational studies, the USPSTF places more emphasis on studies known as randomized clinical trials, according to Wender.

In observational studies, researchers monitor groups of people for a period of time without intervening in their lives in any way, then evaluate the course or frequency of a disease. In randomized clinical trials, researchers randomly assign participants to different interventions (or to an intervention vs. no intervention), then follow them over time to see the effects of the intervention.

The task force identifies preventive measures, such as mammograms, and considers the net benefit, such as lives saved, versus potential harms, such as the emotional stress of being called back for additional testing. It classifies the net benefit of mammograms in women under 50 as small, and those in women over 50 as moderate. "As a result," Wender says, "they tend to be more conservative'' than other organizations.

Related: Find Out Which Screening Tests You Need When With This Chart

Why the guidelines keep changing

It may seem like the guidelines change with the wind, but that's not true, Wender says. The ACS guideline for women at average breast cancer risk hadn’t been updated in 12 years, since 2003.

The USPSTF is currently reviewing its mammography guidelines and has a revision in progress.

The guidelines change as new information and research becomes available and the experts from various organizations review it.

How to decide the best plan for you

Guidelines are written for the general population. To decide the best course of action for yourself, experts say to talk with your doctor. Take into account any family history of breast cancer, your age, how likely you are to become very stressed if you are called back for a repeat test and other factors.

Be careful, too, experts say, not to misinterpret the guidelines. The new ACS guidelines, for instance, say women should have the opportunity to start at age 40 to 44, but that all women should start by age 45. Some women will undoubtedly feel more comfortable getting a mammogram at age 40, while others may feel better beginning at age 45.

Once you and your doctor have made the best decision about mammography for you, expect to revisit the question as you get older. Your desires for screening may increase or decrease, depending on your personal preferences and medical history.

Related: 6 Powerful Ways to Reduce Your Cancer Risk

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.