But some alarming new images might make expectant mothers think twice about continuing a smoking habit during pregnancy.
Using high-definition ultrasound scans, scientists have shown that maternal smoking can alter the mouth and hand movements of the fetus, a finding that suggests some impairment of the fetus' central nervous system development.
The study's lead author, Dr. Nadja Reissland of England's Durham University, hopes these images will one day be used as educational tools to help expectant mothers make healthier choices.
"[Ideally], when we show mothers... these videos of fetuses showing increased movement, they will be more inclined to stop smoking," said Reissland in a video by the Press Association, a U.K. media organization.
Researchers from Durham and Lancaster Universities in the U.K. analyzed 80 high-definition ultrasound scans of 20 fetuses, collected at 24 and 36 weeks into the pregnancy, in order to observe their mouth and hand movements. Four fetuses were being carried by mothers who smoked an average of 14 cigarettes per day, while the other fetuses were carried by mothers who did not smoke.
The researchers saw that the fetuses whose mothers smoked displayed a rate of mouth movements significantly higher than normal. Typically, fetuses do move their mouths and touch themselves, but this movement tends to decline as birth approaches and the fetus gains more control over its motor functions.
Face and mouth movements of a fetus carried by a mother who smoked daily during pregnancy (above) compared to those of a fetus whose mother did not smoke.
The effects of smoking on the fetus's hand and mouth movements are similar to those seen among fetuses whose mothers are highly stressed or depressed. However, the effects of nicotine exposure appear to be considerably more dramatic.
Why? The researchers hypothesize that when a fetus is exposed to nicotine, its central nervous system, which controls facial movements, does not "develop at the same rate and in the same manner" as in normal fetuses, according to a Monday press release from Durham.
The findings were published in the March issue of the journal Acta Paediatrica.