Even casual smoking during pregnancy produces behavioural changes in newborn babies similar to those induced by illegal drugs, research has found.
Nicotine affects unborn babies
Scientists found that women who smoked just six to seven cigarettes per day gave birth to babies who more jittery, more excitable, stiffer and more difficult to console than newborns of non-smokers.
And the higher the dose of nicotine measured in a mother, the greater the signs of stress in her new baby.
We have a legal drug in nicotine that may have the same toxic effect as illegal drugs
The behavioural changes were similar to those found in newborns of women who use crack cocaine or heroin while pregnant - and were strong enough to suggest that babies go through a "nicotine withdrawal" response.
Researcher Karen Law, from Brown University, Providence, Rhode Island, said: "We have a legal drug in nicotine that may have the same toxic effect as illegal drugs.
"It is a huge public health concern that so many people are suffering the costs of smoking, including newborns."
Her colleague Dr Barry Lester said there was far more concern about the use of illegal drugs during pregnancy, than the use of tobacco.
However, up to six times more women smoked during pregnancy than took illegal drugs.
The new findings suggested that greater efforts were needed to focus on the dangers of smoking.
He said: "If a behaviourally vulnerable baby receives attention and care, there is no reason to think that the child won't thrive.
"But we also know that the same baby is at risk for a poor developmental outcome if that child grows up in a stressed, low-income environment, where effects of exposure get exaggerated."
The research focused on 27 babies who were exposed to tobacco smoke in the womb, and 29 who were not.
The "nicotine" infants were abnormally tense and rigid, required more handling and showed greater signs of stress.
The babies' exposure to nicotine was measured by analysing saliva samples from the mothers for traces of a chemical called cotinine, produced when nicotine is broken down by the body.
Professor David Edwards, a neonatologist at Imperial College, London, told BBC News Online further research would be required to draw any firm conclusions about the effect of exposure to tobacco smoke in the womb.
He said that as yet there was no evidence to equate the effect of tobacco exposure to that of cocaine, which constricts the blood vessels and can cause babies to suffer a stroke.
But he said: "We should make every possible attempt to stop mothers smoking during pregnancy."
Gay Sutherland, a psychologist working for the charity Action on Addiction, said only one in four women smokers quit when they are pregnant, and 80% of those who do manage to give up relapse within a year of giving birth, exposing their babies to the dangers of passive smoking.
She said: "Action on Addiction hopes this research will encourage more women to seek help to give up completely while pregnant, for their own health as well of that of their babies."
The research is published in the journal Pediatrics.