Speaking to ITV's Exposure programme, Professor Peter Hepper, said: "If women drink just one unit of alcohol, the baby's breathing and movement stop for up to two hours after that. That's not normal – the baby should be continually active.
The professor, from Queen's University Belfast, carried out a study on the effects of low-level alcohol in pregnancy – the first of its kind in the UK.
He looked at 18-week scans of pregnant women who drank on average two-and-a-half units a week – equivalent to a 200ml glass of wine.
He noticed the foetuses stopped moving and breathing, then they suddenly jumped and turned themselves over.
Prof Hepper said the jolts suggested the babies' brains were not developing properly.
He told the programme, which airs next week: "The only safe [alcohol] level is zero."
The research, which reveals the dangers of just one unit of alcohol, flies in the face of NHS guidelines.
These imply it is safe for pregnant women to continue drinking as long as it is not more than one or two units, once or twice a week.
Dr Mary Mather, a consultant paediatrician, said British women were being 'deprived' of information about the dangers of drinking while pregnant.
In the US, Canada, Australia, New Zealand, Spain, Switzerland and the Netherlands, pregnant women are told not to drink at all.
Sir Al Aynsley-Green, a professor of child health at University College London and a former Children's Commissioner, said: "Exposure to alcohol before birth is the single most important preventable cause of incurable brain damage.
"There is insufficient reliable information that can help women make these important decisions."
The Department of Health said: "Our advice is that pregnant women or women trying to conceive should avoid drinking alcohol. If they do choose to drink, to minimise the risk to the baby, they should not drink more than one to two units of alcohol once or twice a week and should not get drunk."
What do you think about this new advice?
More on Parentdish: The risks of drinking in pregnancy