Pregnant women who smoke may harm their babies' brain development if they turn to e-cigarettes to satisfy their nicotine craving, scientists have warned.
New research suggests that e-cigarette vapour may be as damaging as tobacco smoke to the nervous systems of the foetus or newborn infant.
The early findings, based on studies of mice, show that exposure to volatile chemicals from the devices disrupts the activity of thousands of genes in the developing frontal cortex, the brain region responsible for higher mental functions.
Analysis of the altered gene activity patterns indicated that they could lead to reductions in learning, memory and co-ordination, and increases in hyperactive behaviour.
These are just the sort of neurological effects seen in children whose mothers smoked during pregnancy and who are known to be at risk of attention deficit hyperactivity disorder (ADHD) and learning difficulties.
A further study, still on-going, has shown that older mice exposed to e-cigarettes in the womb or just after birth do indeed appear to be hyperactive, running around at a faster rate than normal.
Lead scientist Professor Judith Zelikoff, from New York University in the US, said: "This is ground-breaking research. What it shows is that there is certainly some concern over the safety of e-cigarettes, particularly in relation to pregnant women or young infants.
"There are potential dangers revealed by these studies indicating a possible impact to the unborn child that may be seen at birth but may occur later in the life of the child.
"Women may be turning to these products as an alternative because they think they're safe. Well, they're not."
Colleague Dana Lauterstein, a PhD student at the university who did much of the work, added: "Most people view e-cigarettes as a safe way to smoke. For women who are pregnant, this could be dangerous. They could unwittingly be endangering their child."
Prof Zelikoff's team started out expecting to see effects from nicotine, which previous research has suggested may have an impact on brain development.
The whole point of e-cigarettes is that they deliver a dose of nicotine minus the highly damaging cocktail of other chemicals found in tobacco.
For this reason they have been touted as a "healthier" alternative for smokers who lack the will to quit, or a cessation aid that can help to wean them off tobacco.
But recent studies have challenged the view that apart from its addictive properties, nicotine on its own is harmless.
And the new research shows that other e-cigarette chemicals besides nicotine have an even bigger effect on developing nervous systems than the tobacco compound.
In the gene activity study, mice exposed to e-cigarette vapour with its nicotine removed experienced the largest number of changes, with some genes being boosted and others suppressed. Females were more affected than males.
Prof Zelikoff said: "That was really surprising. We were shocked. But what people don't realise is that even without nicotine there are many things that are given off when you heat up and vaporise these products."
Two major components in e-cigarettes are propylene glycol and vegetable glycerin. In addition, various flavourings may be added.
The findings were presented at the start of the American Association for the Advancement of Science (AAAS) annual conference in Washington DC, the world's largest general science meeting.
In the first study, 30 to 40 normal pregnant mice were exposed to e-cigarette vapour from the popular "Blu" brand, with or without nicotine, or allowed to breathe ordinary air. Dose levels were "relatively low", said the scientists.
The exposure lasted the full length of the mothers' pregnancies, about three weeks. Four days after birth, their offspring underwent the same exposures for another three weeks.
Tissue samples from the animals' brains were then tested for gene activity. Intelligent software was used to predict the likely effects of the patterns seen, based on the results of previous genetic research.
The second study involved around 60 three-month-old mice.
Activity levels were found to be significantly higher in male and female mice that had been exposed to e-cigarette vapour, with or without nicotine. This time refillable "tank" e-cigarettes were used.
E-cigarette exposure was also associated with greater "vertical time" - how much time the mice spent rearing on two legs, which is linked to exploratory behaviour and curiosity.
The implications of this are not certain but it could indicate a propensity for risk taking, the scientists believe.
Prof Zelikoff challenged the idea that e-cigarettes played a valuable role in helping people quit tobacco.
"Many people who use e-cigarettes are dual users," she said. "They enjoy e-cigarettes and they smoke. And e-cigarettes can be a gateway - there are concerns that adolescents are starting with e-cigarettes and going onto real cigarettes.
"It's not straightforward. There isn't hard evidence that e-cigarettes are working in that regard."
Professor Adam Balen, chairman of the British Fertility Society, said: "Whilst e-cigarettes may help some people to stop smoking real cigarettes, one cannot escape the reality that various chemicals are still being inhaled that have potentially harmful effects both to health, fertility and also the non-consenting participant, that is the baby.
"It may therefore be wrong to switch during pregnancy and best to avoid all kinds of smoking."
In the UK, a hi-tech e-cigarette called Evoke - which can communicate with smartphones - has recently been licensed as a medicine by the Medicines and Healthcare products Regulatory Agency (MHRA).
A spokesman for the agency said: "It is a product of acceptable quality and can be an effective aid to smoking cessation.
"We hope to see more e-cigarettes and next generation nicotine delivery product medicines licensing applications in the future."
Other research by the same US team has suggested that exposure to e-cigarettes may have a harmful effect on male fertility.
In another mouse study, three to five-week-old male offspring whose mothers had been exposed to e-cigarette vapour were found to have lower concentrations of sperm. Their sperm was also significantly less active than non-exposed mice.