Pregnant smokers who take up e-cigarettes as a healthier substitute for tobacco may still be running significant risks with the wellbeing of their unborn children.

Chemicals present in the vapour produced by e-cigarettes can cause developmental problems in laboratory mice whose mothers had been exposed to “vaping” during pregnancy, scientists said.

In Britain, pregnant women are strongly advised to give up smoking altogether, but alternatives to tobacco, such as e-cigarettes, are considered a healthier substitute and are even available on the NHS.

However, medical advice suggesting that vaping is healthier than cigarette smoking does not necessarily mean it is safe for pregnant women, said Professor Judith Zelikoff of New York University, who led the research team.

Tests on mice exposed to low levels of e-cigarette vapour over the course of their pregnancy and during the first few weeks in the lives of their offspring, found significant differences in brain activity, sperm counts and behaviour of their progeny compared to unexposed mice, Professor Zelikoff told the American Association for the Advancement of Science in Washington.

The effects were even more pronounced when nicotine, the active ingredient of e-cigarette vapour, was removed from the vapour, indicating that other chemicals added to the vapour are exerting a significant impact on foetal development.

“Our studies also show that exposure to vapour from electronic cigarettes, in utero and postnatally, drastically reduces sperm counts and sperm mobility in juvenile offspring and brings about gene changes in the brain as well as altered behaviour in adult male and female offspring,” Professor Zelikoff said. “This is groundbreaking research. What it shows is that there is certainly some concern over the safety of e-cigarettes.”