(NaturalNews) If you're a mother-to-be and you happen to enjoy a couple cups of coffee in the morning, you may be undercutting your baby's birth weight.
That's the conclusion by researchers who have conducted a decade-long study of nearly 60,000 women in Norway - expectant mothers who drink two cups of coffee a day risk having an underweight baby and having their pregnancies last longer, though only by a matter of hours, Britain's Telegraph newspaper reported.
The findings included data about how often women had foods or drinks that contained caffeine, including tea and coffee, as well as chocolate sandwich spread and bars of chocolate.
Researchers compared those findings with details of infant birth weight and in doing so established a clear link between caffeine and lower birth weights. Data indicated that consumption of between 200-300 mg of caffeine per day raised the odds of a newborn being classed as small for the length of pregnancy by up to 62 percent.
More than two cups a day especially risky
One average mug of coffee contains about 100 mg of caffeine; a mug of filter coffee contains more, around 140 mg. But some drinks sold in coffee shops - such as espresso and other high-caffeine content drinks - may contain as much as 300 mg per cup.
Other countries, such as Great Britain, recommend limiting caffeine consumption to just 200 mg per day.
Scientists conducting the study also said they specifically identified coffee as increasing the length of pregnancy; one daily average cup of coffee lengthened child labor by as much as eight hours. Unlike previous studies, the new research did not identify a link between caffeine and premature birth.
Dr. Verena Sengpiel of Sahlgrenska University Hospital in Sweden, a primary researcher, said caffeine may stunt the growth of the unborn baby by slowing the vital passage of nutrients from the mother to the infant through the placenta.
Sengpiel, writing in the journal BMC Medicine, added that coffee could make a noticeable increase in the length of a pregnancy by hours, simply by interfering with the chemical signals occurring around the beginning of labor.
"The UK Food Standards Agency carefully analyzed and thoroughly reviewed the effects of caffeine during pregnancy and currently recommends that pregnant women moderate consumption to an upper safe limit of 200 mg per day - two to three cups of coffee," said Dr. Euan Paul of the British Coffee Association.
"Switching to decaf during pregnancy is also an option for those who wish to continue drinking coffee," he said. "We welcome more research into this important area so that the associations found in this study can be further explored."
Earlier research also links caffeine to low birth weights
Annette Briley, a consultant midwife for Tommy's, a baby charity, said low birth weights can lead to opposite problems in the future.
"Being born small can lead to catch-up growth and this in turn can lead to obesity, diabetes and certain cancers in adult life," she told the paper. "While women do need to be mindful and remember that caffeine is found in tea, chocolate, other sweets and soft drinks - as well as coffee - we would suggest further research into the effects of coffee is required."
She added: "Additional care should however be taken when buying coffee in retail outlets as the caffeine content varies between many companies. If women are worried, they should seek advice from their GP or Tommy's midwives for the best advice."
Glasgow University scientists warned in 2011 that pregnant women were at risk of endangering the health of their unborn babies by drinking coffee from high-caffeine shops. They analyzed espressos from 20 notable coffee shops and found substantial variations in the amount of caffeine in each shop's products, with the strongest levels found having six times the caffeine content as the weakest.