New ultrasound pictures show how babies of mothers who smoke during pregnancy touch their mouths and faces much more than babies of non-smoking mothers.
Foetuses normally touch their mouths and faces much less the older and more developed they are.
Experts said the scans show how smoking during pregnancy can mean the development of the baby’s central nervous system is delayed.
Doctors have long urged pregnant women 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.
Dr Nadja Reissland, of Durham University, used 4-D ultrasound scan images to record 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, she explained.
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.