KABUL, Afghanistan — One of the best ways to measure improvement in a nation’s health is the maternal mortality rate, and American officials have long held out Afghanistan’s striking improvement there as an important legacy of Western aid.

In 2002, when maternal health was surveyed soon after the American-led invasion that ousted the Taliban, the mortality rate was 1,600, meaning that 1,600 women died for every 100,000 live births — a rate comparable to the Middle Ages in Europe. In 2010, a study financed by the United States Agency for International Development determined that Afghanistan’s maternal mortality rate had dropped to 327, a stunning improvement.

Now a group of experts in the maternal mortality field have come along and said, in effect, that if it sounds too good to be true, it is.

In a paper commissioned by the British and Irish Agencies Afghanistan Group, an organization representing aid groups from the British Isles, researchers from Harvard, the University of Aberdeen, the Melbourne School of Population and Global Health, and HealthProm, a charity based in Britain, compared the two studies and found the improvement in maternal health implausible.