Even though it has long been accepted that smoking during pregnancy is discouraged, new ultrasound scans have further demonstrated this by showing unborn babies appearing to grimace in their mother’s womb after they light up.

The images came from a U.K.-based research project led by Dr. Nadja Reissland, whose findings were published in the journal Acta Paediatrica. Reissland monitored 20 pregnant women attending the James Cook University Hospital in Middlesbrough, four of whom smoked an average of 14 cigarettes per day. She studied their 4D scan images at 24, 28, 32, and 36 weeks, as well as recording thousands of tiny movements in the womb.

Reissland and her colleagues found that the fetuses of the mothers who smoked showed much higher rates of self-touching and mouth movements than those of the mothers who didn’t smoke. The observation was noteworthy because fetuses typically move their mouths and touch themselves less as they get closer to birth due to gaining more control of their bodies. Reissland concluded that the babies of these smokers may have suffered delayed developments in their central nervous system.

"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,” said co-author Brian Francis, a professor at Lancaster University. “This is further evidence of the negative effects of smoking in pregnancy.”

Reissland’s findings added on to the list of already-proven negative consequences of smoking during pregnancy, including a reduced egg quality and increased chances of premature birth, respiratory problems, and cot death. However, she was quick to call for support for mothers in similar circumstances to help them quit smoking instead of demonizing them.

Although 2014 showed that the number of British women who smoke during their pregnancy fell to its lowest percentage in eight years, it still remains an issue for many mothers. Approximately 12% of pregnant women in the U.K. smoke, but that number is as high as 20% in some countries. The Department of Health is hoping to reduce the overall number to 11% by the end of the year.