A US task force has come to the conclusion mammograms do most good when women have them from the age of 50 onwards and should be merely an option at 40.

Mammograms do the most good later in life, a US government task force has declared in recommending that women get one every other year starting at age 50. 

It says 40-somethings should make their own choice after weighing the pros and cons. 

When to start routine mammograms and how frequently to get them has long been controversial.

The latest guidelines from the US Preventive Services Task Force stick with its advice that women should have one every two years between ages 50 and 74.

But they also make clear that it's an option for younger women even though they're less likely to benefit.

Some health groups urge mammograms every year starting at 40 - although last year the American Cancer Society upped its starting age to 45.

There is some common ground emerging, that mammography advice shouldn't always be one-size-fits all.

"Age 50 isn't magic," task force past chairman Dr Michael LeFevre of the University of Missouri said on Monday.

Monday's update, published in Annals of Internal Medicine, is largely a rewording of guidelines originally issued in 2009 and reconsidered in draft form last spring.

This time, the task force stresses that "we think the science supports a range of options" for 40-somethings, LeFevre said.

SOME THINGS TO KNOW ABOUT MAMMOGRAMS:
WHAT THE TASK FORCE SAYS
Women in their 60s are the most likely to avoid dying from breast cancer thanks to mammograms, but there's clearly enough benefit for the average woman to start at 50, the task force found. The advisory group wants younger women to understand the trade-offs before deciding: Among every 1000 women screened, one additional death could be prevented by starting mammograms at 40 instead of 50. But there would be 576 more false alarms and 58 additional unneeded biopsies.

DIFFERING GUIDELINES
Mammograms aren't perfect, and different health organisations weigh the trade-offs differently. The American Cancer Society says to begin annual mammograms at 45 but switch to every other year at 55. After menopause, tumours tend to grow more slowly and women's breast tissue becomes less dense and easier for mammograms to penetrate, says chief medical officer Dr Otis Brawley.

THINGS TO CONSIDER
In the US more than 200,000 women are diagnosed with breast cancer each year, and about 40,000 die from it. It is most frequently diagnosed among women ages 55 to 64, and the median age of death from breast cancer is 68.

WHEN TO STOP
The task force says more research is needed to know whether to continue mammograms at 75 and beyond. The cancer society says to keep screening as long as women are in good health and have a life expectancy of at least 10 years.

WHAT'S NEXT
The task force said more research is needed to tell if newer 3-D mammograms should be used for routine screening and if women with dense breasts benefit from extra testing, such as with ultrasounds or MRIs.