Research shows smoking during pregnancy harmed foetuses, The Telegraph reported.
The series of 4-D ultrasound scans, captured by researchers from Durham and Lancaster universities, revealed how babies in womb opened their mouths, covered their faces, and appeared to "grimace" when their mothers inhaled cigarette smoke.
According to the study, fetuses whose mothers smoked, showed a significantly higher rate of mouth movements than the normal declining rate of movements expected in a fetus during pregnancy.
Though further research would be needed to confirm the results, according to professor Brian Francis, co-author of the study, technology had revealed what was hidden earlier, and that smoking affected the development of the foetus in ways we did not realise.
He concluded that the research was yet further evidence of the ''negative effects of smoking in pregnancy."
The small pilot study was conducted on 20 pregnant women - 16 non-smokers and four smokers who underwent ultrasound scans to observe fetal activities at 24, 28, 32 and 36 weeks.
According to the study published in journal n , the four fetuses of smoking mothers covered their faces and moved their mouths more than those of non-smoking mothers.
Foetuses touched themselves and moved within the womb, but as they grew older their movements decreased, lead researcher Nadja Reissland of Durham University in the UK said.
Reissland said all the foetuses had a healthy birth weight but the images showed that smoking affected "fine grain behaviors."
According to Reissland, who spoke to USA Today Network, normal foetal behaviour showed fewer movements but the images suggested that foetuses in smokers were less mature in their behaviour.
According to the CDC, one in five babies born to mothers who smoked had a low birth weight. Also smoking during pregnancy was more likely to result in preterm delivery and other health issues.
While rates of smoking among pregnant women had dropped in the US, 13.8 per cent smoked while pregnant according to the CDC.
Reissland said she wanted to replicate the results in a larger study with 60 smokers and 60 non-smokers. She hoped the study would one day become a common practice to show mothers and families the effects of smoking on foetuses in the womb.
(See: High-definition scans suggest effects of smoking may be seen in unborn babies)