Pregnant mothers should avoid consuming coffee from high street cafes as they contain more caffeine than recommended levels, researchers say.
The soon-to-be mothers inadvertently put the health of their unborn babies at risk by drinking coffee from such cafes.
The researchers analysed espressos from 20 coffee shops and found that there were huge variations in the amount of caffeine they contain - with the strongest having more than six times as much as the weakest.
The most worrying part was that the strongest example contained over 50 per cent more caffeine in a single cup than a pregnant woman is advised to take over the course of an entire day.
Fears about birth defects, miscarriage and premature delivery imply that pregnant women are advised to limit themselves to 200mg of caffeine a day.
This is generally equivalent to four cups of strong coffee, expected to contain 50mg of caffeine each.
But only one of the 20 coffees studied - the one from Starbucks - contained around this amount. All of the others were stronger, the Daily Mail reported.
According to the Royal Society of Chemistry journal Food and Function, four had more than 200mg in a single cup, with the strongest having more than 300mg.
Researcher Alan Crozier said that caffeine, which crosses the placenta to the unborn baby, usually takes five hours to break down in the body to half its previous level.
"There is clearly a problem. People at risk could unwittingly ingest far more caffeine than they would ever dream of," he said.
But the process can take up to 30 hours in certain groups, including children, liver disease sufferers and pregnant women.
An unborn baby's liver will struggle to break down caffeine, with potential long-term consequences.