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 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 they could lead to reductions in learning, memory and co-ordination, and increases in hyperactive behaviour.
Those are 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 continuing, has shown that older mice exposed to e-cigarettes in the womb or just after birth 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."
E-cigarettes, which deliver a dose of nicotine minus the damaging other chemicals found in tobacco, 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, some genes being boosted and others suppressed. Females were more affected than males.
Prof. Zelikoff said: "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.
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."
Other research by the 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.