Women are being advised to cut out caffeine during the first three months of pregnancy, to help reduce their risk of having a miscarriage.

The warning follows a study that found pregnant women who consumed two or more mugs of coffee a day were twice as likely to miscarry than those who abstained from caffeine completely.

The study of 1,063 pregnant women showed that those who had at least 200mg of caffeine each day had a 25% risk of miscarriage compared with a 12% risk for women who avoided all caffeine, including in tea, soft drinks and chocolate.

The advice, from doctors at one of America's largest private health organisations, goes further than guidelines introduced by British government health officials in 2001.

These recommend that pregnant women limit their daily intake to 300mg.

The government advice was based on a Food Standards Agency review of the scientific literature, which found an increased risk where caffeine intake was highter than 300mg a day - or three cups of instant coffee - in the first 15 weeks. Fresh coffee is typically stronger than instant, and a cup of tea has around 40mg.

"It appears that 200mg a day is already a high enough dose to increase the risk of miscarriage," said Dr De-Kun Li, a perinatal epidemiologist who led the study at Kaiser Permanente, a California-based non-profit health foundation that runs 32 hospitals. He advised women to consider not consuming caffeine or at least limiting their intake to less than 200mg during the first 12 weeks of pregnancy.

The women were asked what they had eaten and drunk until the 20th week of gestation: 635 had up to 200mg a day; 164 women had 200mg or more daily; and 264 had none.

The doctors accounted for other factors known to affect miscarriage rates, including age, income, a history of miscarriages, smoking and alcohol consumption. Overall, 172 women miscarried, 95% within the first 15 weeks of pregnancy.

The results suggested that even moderate amounts of caffeine increased the risk, but this was only confirmed in women who consumed 200mg or more a day. It made no difference whether the caffeine came from coffee, tea, chocolate or cola drinks. Li called on GPs to encourage patients to reduce or give up caffeine during early pregnancy.

It is unclear why caffeine should affect early pregnancy, although it is known that pregnant women take longer to process caffeine and it can cross the placenta into a baby's bloodstream.

The latest study, in the current online issue of the American Journal of Obstetrics and Gynaecology, accounted for this "pregnancy-related caffeine aversion" - which Li said had hampered previous studies - by following women who did not change their drinking habits.

High levels of caffeine have previously been linked to increased risk of premature birth and small babies. But a Danish study last year found no difference where mothers had drunk only moderate amounts of caffeinated and decaffeinated coffee.

Tracy Flanagan, director of women's health at Kaiser Permanente in northern California, advised pregnant women to consider switching to decaffeinated drinks, and natural energy boosts, such as a brisk walk, yoga or dried fruit.

A spokeswoman for the Department of Health said: "We are constantly reviewing and updating our advice, but at this time, our guideline limit of 300mg of caffeine remains." The guidelines also suggest pregnant women try decaffeinated drinks where possible.

The effects

Caffeine occurs naturally in a range of foods, such as coffee, tea and chocolate, and is added to soft drinks such as Red Bull. Cold remedies and antihistamines use it to counter the drowsy effects of other ingredients. Caffeine raises the heart rate and metabolism. It is a diuretic that causes the body to lose water, other fluids and calcium. Tea hinders the ability to absorb iron if drunk near a meal. Babies find it hard to excrete.