"We have known for 50 years that infants born to mothers who smoke during pregnancy have lower birth weights," lead investigator Dr Malene Rohr Andersen, from Gentoffe University Hospital in Hellerup, Denmark, said in a statement. "But this study provides a possible explanation of why there is restricted blood flow to the foetus," she added.

Andersen's team studied 266 pregnant women, of which 182 were non-smokers, 43 were smokers, and 41 were ex-smokers.

According to a report in the medical journal Circulation, compared with newborns of non-smokers and ex-smokers, those of smokers weighed less, had smaller head sizes, and were shorter.

Smoking was associated with a 47 per cent drop in the foetus in levels of a key protein that helps blood vessels to relax and blood flow to increase called endothelial nitric oxide synthase (eNOS). "This study is one of the first to show a biochemical measurement of what is going on to cause the lower birth weight," Andersen said.

Smoking was also associated with an 18 per cent reduction in plasma levels of "good" HDL cholesterol in the foetus.