A new study may give pregnant women who smoke a new incentive to quit: 4-D ultrasounds show that the habit is causing their unborn babies to grimace. 

Researchers at Durham and Lancaster Universities, in the United Kingdom, studied high-definition ultrasound scans and noticed that the fetuses of mothers who smoked had more mouth movements than expected during pregnancy.

“Technology means we can now see what was previously hidden, revealing how smoking affects the development of the fetus in ways we did not realize,” study co-author Brian Francis, a professor at Lancaster University, said in a news release. “This is yet further evidence of the negative effects of smoking in pregnancy.”

Study authors hypothesized that those increased movements may be tied to central nervous system function, which controls movement. The fetuses of the mothers who smoked didn’t follow a normal developmental process compared to the fetuses of the mothers who didn’t smoke. Previous research also suggests that exposure to cigarette smoke in the womb is associated with delayed speech development in infants.

The study’s preliminary findings stem from an analysis of 80 4-D ultrasound scans of 20 healthy fetuses, four of which were carried by mothers who smoked an average of 14 cigarettes per day, while the remaining 16 were carried by nonsmokers. Researchers took the scans at four different times between 24 and 36 weeks of pregnancy, and observed that maternal smoking was associated with greater fetal mouth and touch movements.

“Our findings concur with others that stress and depression have a significant impact on fetal movements, and need to be controlled for. But additionally,  these results point to the fact that nicotine exposure per se has an effect on fetal development over and above the effects of stress and depression,” lead study author Dr. Nadja Reissland, a psychology professor at Durham University, said in the news release. “A larger study is needed to confirm these results and to investigate specific effects, including the interaction of maternal stress and smoking.”

Study authors said future studies also may analyze how fathers’ smoking behaviors may influence fetuses developmentally.

The study is published in the journal Acta Paediatrica.