Smoking has long been known to be harmful during pregnancy, increasing the risk of miscarriage, premature birth, and other problems. Now, it seems the harmful effects of smoking during pregnancy may actually be visible by ultrasound, according to new research from Lancaster and Durham Universities in the U.K.

Based on 4D ultrasound images, the researchers found that fetuses carried by mothers who smoked were more likely to touch their face and have mouth movements than those of non-smoking women, a sign of delayed development.

“Normally as the fetus matures, they show more complex movements, and also fewer movements,” Nadja Reissland, PhD, associate professor of psychology at Durham University and lead author of the study told Health. “The frequency of movements should slow as the fetus grows older, so these images indicate that there might be some sort of delay in maturation.”

The fetuses of smokers had 58% more mouth movements than those of non-smokers at 30 weeks, Reissland said. Their mouth movements also declined more slowly: just a 1.5% decrease for each additional week in the womb, about half the decline of fetuses belonging to non-smokers.

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Reissland believes that nicotine may be affecting brain development in the fetuses, which slows their growth. “It could be because the nicotine in cigarettes binds to receptors in the brain, and that leads ultimately to premature brain cell death,” she says. “So it could be that the fetuses of smoking mothers have more dying brain cells, and therefore they show more immature behavior.”

While all of the fetuses of smoking mothers were born at a healthy weight and age—that is, not prematurely—she believes these images may indicate unseen neurological issues.

“They were declared healthy at their 20-week anomaly scan, and they were born with the right gestational age, the right weight, so they seem to be healthy babies,” Reissland explained. “But still, with my analysis, I can show there’s a difference in their behavior. They appear healthy, but in fact might already have some sort of damage. And it’s most likely central nervous system damage.”

Reissland emphasized that this is just a pilot study, and that she’d like to continue her research with a larger sample size.

“What we need now is a statistically-powered study with around 60 mothers who are smoking and 60 non-smokers, in the second and third trimesters of pregnancy," she said. "And then I'd like to follow up after [the birth] to see what differences in the behavior of the children exist."

In the study, Reissland and her colleagues took images of 20 fetuses at four intervals from the 24th to the 36th weeks of pregnancy. Four of the fetuses belonged to mothers who smoked an average of 14 cigarettes a day, while the other 16 fetuses were carried by non-smokers.

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