The report suggests that drinking 200-300 milligrams of caffeine per day raised the risk of a baby being born small by between 27% and 62%.
Smaller babies have higher risks of certain health problems, and the researchers say recommendations on safe limits need to be reconsidered.
Baby Eats What Mom Eats
Everything the mom-to-be eats and drinks potentially gets through to the growing baby. That's why health organizations set recommendations for limits on things like alcohol and caffeine in pregnancy.
Along with nutrients and oxygen, caffeine passes from the placenta to the baby. However, the effect isn't the same on a baby as it is in adults.
The American College of Obstetricians and Gynecologists recommends limiting caffeine to 200 milligrams per day. That’s equal to two 8-ounce cups of brewed coffee. Caffeine levels may be stronger in coffee bought at a coffee house.
Caffeine is also found in tea, colas, energy drinks, and chocolate.
For the new research, published in the journal BMC Medicine, researchers looked at information about diets and birth details of about 60,000 pregnancies over 10 years.
All sources of caffeine were monitored in the study.
In a news release, researcher Verena Sengpiel, MD, from Sahlgrenska University Hospital in Sweden, says, "In this study we found no association between either total caffeine or coffee caffeine and preterm delivery, but we did find an association between caffeine and SGA [small for gestational age]."
The source of caffeine also seemed to make a difference. Women who daily had 100 milligrams of caffeine in general increased the length of their pregnancy by five hours. However, caffeine from coffee was found to add eight hours overall from drinking 100 milligrams a day.
Sengpiel says the study shows that the guidelines should be looked at again.
So what should a coffee-loving pregnant woman with concerns do?
"Good advice would be to keep it to a minimum,” says Pat O’Brien, MD, a spokesman for the Royal College of Obstetricians and Gynaecologists in the U.K. “One or a maximum of two cups a day."