Drinking decaffeinated coffee could increase the risk of heart disease, a study has suggested.
Experts say pregnant women can drink a small amount of coffee
It could lead to a rise in harmful cholesterol levels, the US National Institutes of Health study found.
The finding comes as a Danish team reiterated that drinking eight or more cups of coffee a day while pregnant may double the risk of losing the baby.
They advised pregnant women to drink no more than three cups of coffee a day, in line with existing UK advice.
The US study looked at 187 people, a third of whom drank three to six cups of caffeinated coffee a day, while a second group drank the same amount of decaffeinated coffee, and the rest had no coffee.
Researchers measured the level of caffeine in people's blood, as well as a number of heart-health indicators, including blood pressure, heart rate and cholesterol levels over the course of the three month study.
At the end of the study, the group drinking decaffeinated coffee had experienced an 18% rise in their fatty acids in the blood, which can drive the production of bad 'LDL' cholesterol.
Fatty acids did not change in the other groups.
Having a high level of LDL cholesterol is one of the risk factors for metabolic syndrome, which can lead on to heart disease and diabetes.
In addition, a protein linked to bad cholesterol (apolipoprotein B),went up 8% in the decaffeinated group but did not significantly change in the other two groups.
The research was presented to a meeting of the American Heart Association.
Dr Robert Superko of the Fuqua Heart Centre in Atlanta, Georgia, who led the research, said: "Contrary to what people have thought for many years, I believe it's not caffeinated but decaffeinated coffee that might promote heart disease risk factors."
But he added: "If you only drink one cup each day, the results of our study probably have little relevance because at that level your daily coffee dose is relatively low."
Judy O'Sullivan, of the British Heart Foundation said: "As the study was quite small and short-term it is too soon to draw any firm conclusions about the use of coffee to reduce risk of heart disease.
"Additionally, it examined the effects of drinking three to six cups of black decaffeinated and caffeinated coffee daily. Therefore, it is not relevant for those who enjoy a coffee once or twice a day.
The second study, from a team at Aarhus University in Denmark monitored over 88,000 pregnant women between March 1996 to November 2002.
All women were interviewed about potential risk factors which could affect their pregnancy, and coffee consumption.
Just over 3% of women (3,018) drank eight or more cups of coffee a day.
Sixty-seven, out of a total of 1,102 foetal deaths, were seen in this group.
These women were more likely to smoke and drink higher levels of alcohol - both factors which are suspected to increase miscarriage risk.
However, the researchers say they took this information into account when collating their results.
They also suggest that, as high tea and cola consumption was not linked to an increased risk of miscarriage or stillbirth, it could be chemicals in coffee - rather than caffeine.
The research comes to the same conclusion as an earlier smaller study by the same team.
The UK's Food Standards Agency advises pregnant women to drink no more than 300mg of caffeine a day, equivalent to three cups of coffee or eight cups of cola.
Claire Friars, a midwife with Tommy's, which funds research into miscarriage and stillbirth, said: "The results seem to show an increased risk of foetal death from increased coffee consumption in pregnancy, so pregnant women may want to review the amount of coffee they drink whilst pregnant."
A spokeswoman for the British Coffee Association said both studies showed moderate coffee drinking was "perfectly safe".
She added: "There are hundreds of studies which do not show increased health risks associated with drinking caffeinated, and particularly decaffeinated, coffee."