Banning smoking in enclosed public places can lead to lower rates of preterm birth, according to Belgian researchers who say the findings point to health benefits of smoke-free laws even in very early life.

It is well known that smoking during pregnancy can stunt the growth of unborn babies and shorten gestation, and that second-hand smoke exposure can also effect births, but little was known about the impact of smoking bans on preterm birth rates.

So a team of researchers led by Tim Nawrot of Belgium's Hasselt University investigated trends in preterm births - before 37 weeks gestation - from 2002 to 2011 covering a period before, during and after the introduction of smoke-free laws.

They found the risk of preterm birth after the introduction of each phase of Belgium's smoking ban, which was implemented in three phases - in public places and most workplaces in January 2006, in restaurants in January 2007, and in bars serving food in January 2010.

No decreasing trend in preterm was evident in the years or months before the bans, the researchers said in their study in the British Medical Journal on Friday.

"Our study shows a consistent pattern of reduction in the risk of preterm delivery with successive population interventions to restrict smoking," the researchers wrote.

"It supports the notion that smoking bans have public health benefits even from early life."

Smoking causes lung cancer, often fatal, and other chronic respiratory diseases. It is also a major risk factor for cardiovascular diseases, the world's number one killers.