March 25 (UPI) -- Pregnant women who drink caffeine-containing beverages -- even a half a cup of coffee per day -- deliver slightly smaller babies compared with those who avoid caffeinated beverages entirely, according to a study published Thursday by JAMA Network Open.
Compared to infants born to women with no or minimal blood levels of caffeine, infants born to those with the highest blood levels of caffeine were 3 ounces lighter and just under a quarter of an inch shorter, the data showed.
In addition, the babies of women with high blood caffeine levels had head circumferences approximately one-tenth of an inch smaller than those of women who did not consume caffeine during pregnancy.
"Until we learn more, our results suggest it might be prudent to limit or forego caffeine-containing beverages during pregnancy," study co-author Dr. Katherine L. Grantz said in a press release.
"It's also a good idea for women to consult their physicians about caffeine consumption during pregnancy," said Grantz, a researcher with the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
Smaller birth size can place infants at higher risk of obesity, heart disease and diabetes later in life, according to the National Institutes of Health.
Caffeine is a stimulant found in coffee and many soft drinks, and high consumption in most cases is defined as more than 200 milligrams per day, the researchers said.
Caffeine is believed to cause blood vessels in the uterus and placenta to constrict, which could reduce the blood supply to the fetus and inhibit growth. It also may disrupt fetal stress hormones, putting infants at risk for rapid weight gain after birth and for later life health problems.
A study published in January by the International Journal of Obesity linked high caffeine consumption during pregnancy to infants being small for their stage of pregnancy, while others have found similar elevated risks for low birth weight and other poor birth outcomes.
A typical cup of coffee contains about 95 milligrams of caffeine, according to the Mayo Clinic.
For this study, the NIH researchers analyzed data on more than 2,000 women at 12 clinics who were enrolled from 8 to 13 weeks of pregnancy onward.
All of the study participants were non-smokers and did not have any health problems prior to pregnancy.
From weeks 10 to 13 of pregnancy, the women provided a blood sample that was later assessed for levels of caffeine and paraxanthine, a compound produced when caffeine is broken down in the body.
Study participants also reported their daily consumption of caffeinated beverages, including coffee, tea, soda and energy drinks, periodically throughout their pregnancies.
Based on the women's own estimates of the beverages they drank, those who consumed about 50 milligrams of caffeine a day -- or about half a cup of coffee -- delivered infants about 2.3 ounces lighter, on average, than infants born to non-caffeine consumers, the data showed.
"Our results suggest that caffeine consumption during pregnancy, even at levels much lower than the recommended 200 mg. per day of caffeine, may be associated with decreased fetal growth," Grantz and her colleagues wrote in the study.
"Even small increases in plasma caffeine concentrations and its major metabolite paraxanthine were associated with lower birth weight," they said.