Caffeine content can vary with brands and brewing methods

Currently, the Food Standards Agency sets an upper limit during pregnancy of 300mg - or four cups of coffee a day.

But an American Journal of Obstetrics and Gynaecology study found more than 200mg of caffeine a day doubled the risk compared to abstainers.

Experts said they would review the data to see if advice needed changing.

Women probably should consider stopping caffeine consumption during pregnancy Study author Dr Li

Pat O'Brien, consultant obstetrician and spokesman for the Royal College of Obstetricians and Gynaecologists, said based on the findings he would now be advising women in their first 12 weeks of pregnancy to abstain from caffeine altogether.

"The first 12 weeks is a very vulnerable time for the baby. It's when most miscarriages occur," he explained.

He said most women in early pregnancy went off the taste of caffeinated drinks anyway and so should not find abstaining from them too difficult.

But he said it was unclear whether pregnant women needed to avoid caffeine in later pregnancy.

Miscarriage risk

An estimated one in five pregnancies in the UK will end in miscarriage, affecting around 250,000 women in the UK every year.

There are a number of well-established risk factors, such as increased maternal age, a previous history of miscarriage, and infertility.

But the causes of the majority of miscarriages are not fully understood.

Caffeine has been mooted as a risk factor before, but studies have yielded conflicting results.

For the latest research, Dr De-Kun Li and colleagues at the Kaiser Permanente Division of Research, studied 1,063 women who had become pregnant in the last month or two.

300 mg of caffeine is roughly equivalent to: Four average cups or three average mugs of instant coffee Three average cups of brewed coffee Six average cups of tea Eight cans of regular cola drinks Four cans of so-called "energy" drinks 400g (eight standard 50g bars) of normal chocolate Caffeine content in a cup of tea or coffee varies by different brands and brewing methods Source: Food Standards Agency

They asked the women to provide a detailed diary about their caffeine intake up to their 20th week of pregnancy.

When they compared this information with how many of the women had miscarried by 20 weeks gestation, 172 of the women in total, they found a link.

Compared with non-users, women who consumed up to 200mg of caffeine a day had an increased risk of miscarriage - 15% versus 12%.

For women who drank more than 200mg, the risk increased to 25%.

Abstinence

The increased risk appeared to be related to the caffeine itself, rather than other coffee ingredients because other caffeinated beverages such as tea and hot chocolate showed a similar trend to coffee.

Caffeine is able to cross the placenta to the foetus, but it is not clear what affect this has on the growing baby.

Dr Li said: "The main message for pregnant women from these findings is that they probably should consider stopping caffeine consumption during pregnancy."

Zoe Wheeldon from the British Coffee Association said the latest research, although well designed and robust, did not change the current Food Standards Agency recommendation of a safe upper limit of 300mg of caffeine per day for pregnant women.

"This evidence must be reviewed in conjunction with existing research and it is important to review all the available data rather than taking one study in isolation."

A spokesman from the Food Standards Agency said: "In order to provide a more robust basis for the FSA's advice to pregnant women on caffeine consumption, an FSA-funded study, involving around 2,500 pregnant women, was commissioned in 2003.

"This is almost complete and the results will be presented to the Committee On Toxicity in closed session for consideration.

"We will ask the committee to also look at this new American study. When the committee has reached conclusions the agency will, if appropriate, revise its advice on caffeine consumption in pregnancy."