Unborn babies appear to grimace in the womb when their mother lights up, scientists have shown, demonstrating the harmful effects of smoking during pregnancy.
The tiny movements were captured in the faces of the foetuses during using 4D ultrasound scans.
Pregnant women have long been urged to give up cigarettes because they heighten the risk of premature birth, respiratory problems and even cot death.
Now researchers believe they can show the effects of smoking on babies in the womb - and use the images to encourage mothers who are struggling to give up.
Dr Nadja Reissland has studied moving 4D scan images and recorded thousands of tiny movements in the womb.
She monitored 20 mothers attending the James Cook University Hospital in Middlesbrough, four of whom smoked an average of 14 cigarettes a day.
After studying their scans at 24, 28, 32 and 36 weeks, she detected that foetuses whose mothers smoked continued to show significantly higher rates of mouth movement and self-touching than those carried by non-smokers.
Foetuses usually move their mouths and touch themselves less as they gain more control the closer they get to birth.
The pilot study, which Dr Reissland hopes to expand with a bigger sample, indicated that babies carried by smokers may have delayed development of the central nervous system.
The research, conducted by Durham and Lancaster University, is published in the journal Acta Paediatrica.
Dr Reissland, from Durham's Psychology Department, said: "A larger study is needed to confirm these results and to investigate specific effects, including the interaction of maternal stress and smoking."
She believed that videos of the difference in pre-birth development could help mothers give up smoking.
But she was against demonising mothers and called for more support for them to give up. Currently, 12 per cent of pregnant women in the UK smoke but the rate is over 20 per cent in the Durham, Darlington and Tees area.
All the babies in her study were born healthy, and were of normal size and weight.
Dr Reissland, who has an expertise in studying foetal development, thanked the mothers who took part in her study, especially those who smoked.
"I'm really grateful, they did a good thing," she said. "These are special people and they overcame the stigma to help others."
Co-author Professor Brian Francis, of Lancaster University, added: "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."
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