Deaths from coronary heart disease -- the most common cause of death in the U.S., the U.K. and Canada -- have generally been on the decline since the 1970s.

But researchers from the U.K. said that women under the age of 50 are seeing a rise in deaths.

High levels of smoking, increasing obesity and a lack of exercise could all be contributing to the trend, they said.

Coronary heart disease is when the arteries supplying blood to the heart narrow.

Steven Allender from the University of Oxford studied information on all deaths in England and Wales between 1931 and 2005. They detected a leveling off -- and perhaps even a reversal -- of the rate of decline in CHD mortality in women under 50.

The authors also found evidence that significant advances made in terms of CHD mortality among older populations are not being made in people under the age of 60.