A new study, using 4D scans, claims to show the further harmful effects of smoking on babies in the womb.
Researchers hope to use the findings to encourage mothers who are struggling to quit smoking.
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The study, by Durham and Lancaster University, involved 20 pregnant women, four of whom smoked an average of 14 cigarettes a day.
Lead researcher Dr Nadja Reissland recorded thousands of tiny movements in the womb using 4D scan images.
After studying the scans at 24, 28, 32 and 36 weeks, Reissland found that babies of smokers move their mouths and touch themselves significantly more than those carried by non-smokers.
This suggests that babies carried by smokers may have delayed development of the central nervous system.
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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.
According to co-author Professor Brian Francis, advances in technology mean "we can now see what was previously hidden, revealing how smoking affects the development of the foetus in even further ways we did not realise."