Researchers at Lancaster and Durham universities said the findings of their pilot study added weight to existing evidence that smoking is harmful to foetuses as they develop in the womb and warranted further investigation.

Professor Brian Francis, of Lancaster University, said: “Technology means we can now see what was previously hidden, revealing how smoking affects the development of the foetus in ways we did not realise. This is yet further evidence of the negative effects of smoking in pregnancy.”

Observing 4-d ultrasound scans, the researchers found that foetuses whose mothers were smokers showed a significantly higher rate of mouth movements than the normal declining rate of movements expected in a foetus during pregnancy.

The researchers suggested that the reason for this might be that the foetal central nervous system, which controls movements in general and facial movements in particular did not develop at the same rate and in the same manner as in foetuses of mothers who did not smoke during pregnancy.

Previous studies have reported a delay in relation to speech processing abilities in infants exposed to smoking during pregnancy, the researchers added.

The researchers observed 80 4-d ultrasound scans of 20 foetuses, to assess subtle mouth and touch movements. Scans were taken at four different intervals between 24 and 36 weeks of pregnancy.

Four of the foetuses belonged to mothers who smoked an average of 14 cigarettes per day, while the remaining 16 foetuses were being carried by mothers who were non-smokers. All foetuses were clinically assessed and were healthy when born.

In common with other studies, the research also showed that maternal stress and depression have a significant impact on foetal movements, but that the increase in mouth and touch movements was even higher in babies whose mothers smoked.

The study also found some evidence of a bigger delay in the reduction of facial touching by foetuses whose mothers smoked, compared to the foetuses of non-smokers, but the researchers said this delay was less significant.

The research is published in the journal Acta Paediatrica.

Lead author Dr Nadja Reissland from Durham University said: “foetal facial movement patterns differ significantly between foetuses of mothers who smoked compared to those of mothers who didn’t smoke.

“Our findings concur with others that stress and depression have a significant impact on foetal movements, and need to be controlled for, but additionally these results point to the fact that nicotine exposure per se has an effect on foetal development over and above the effects of stress and depression.

“A larger study is needed to confirm these results and to investigate specific effects, including the interaction of maternal stress and smoking.”

The researchers stressed that their research was a pilot study and that larger studies were needed to confirm and further understand the relationship between maternal smoking, stress, depression and foetal development.

They added that future studies should also take into account the smoking behaviours of fathers.