Coffee shop caffeine levels 'vary widely'
High Street coffee shops could pose a risk to pregnant women because of big variations in the amount of caffeine in each cup, research suggests.
Analysis of espresso coffee from 20 shops found that one was six times stronger than others.
The Food Standards Agency (FSA) says too much caffeine can result in miscarriage or a low birth weight.
Researchers from Glasgow University tested caffeine levels in espressos bought from High Street coffee shops.
The FSA currently advises pregnant women to consume no more than 200mg of caffeine a day, based on an assumption that an espresso contains about 50mg of caffeine. The guideline for an average healthy person is 300mg a day.
The work was led by Alan Crozier, senior research fellow in the faculty of medicine.
"The analysis that we did showed the amount of caffeine ranged from 50mg per cup from Starbucks, up to over 300mg per cup from another coffee house, Patisserie Francoise," he said. "I was very surprised by this."
Drinking just one of the strongest cups of coffee would more than exceed the "safe" level for pregnant women.
"If you go to different coffee houses you can tell that some are much stronger than others," Mr Crozier said.
"The problem comes with people who should have a limited caffeine intake.
"You metabolise caffeine much more slowly when you're pregnant and people with liver disease do not have the enzymes to metabolise caffeine.
"Our data shows that you can have the recommended amount - and more - from just one shot of espresso."
The researchers point out that despite the increasing number of coffee shops on the High Street there is no information about the caffeine contents of various types of coffee.
They add that, although they only tested espresso coffees, many other types of drinks such as lattes and cappuccinos are prepared using single or double shots of espresso.
The study has been published in the Royal Society of Chemistry's journal Food and Function.