Even a Little Coffee in Pregnancy May Impact Newborn’s Weight

“This reduction in birth weight is within the range we see in reductions of birth weight among women who smoke during pregnancy,” Gleason said, noting that smokers tend to deliver babies an average 1.8 to 7 ounces lighter than those of nonsmokers.

The findings were published online March 25 in JAMA Network Open.

But while these results are concerning, pregnant women shouldn’t rush to throw out all their coffee beans, tea bags and diet colas, said Dr. Jill Berkin, an assistant professor of maternal-fetal medicine with the Mount Sinai Health System in New York City.

The results of this study conflict with prior research, which found no significant link between caffeine and fetal growth, Berkin said.

Further, the effects of caffeine on birth size and weight observed here were not enormous, Berkin said, and so it’s hard to say whether these babies would suffer any of the long-term health effects typically associated with stunted fetal development.

These effects can include increased risk of obesity, heart disease and diabetes later in life, the researchers said in background notes.

“It was so very small, really only coming out to about 3 ounces of difference in body weight. Whether the 3 ounces has clinical impact on a baby long-term remains to be determined,” Berkin said. “We know there are poorer outcomes associated with babies that are in the less than tenth percentile for expected weight for gestational age, but not smaller reductions in potential fetal weight, so whether that’s clinically significant is really unknown.”

Berkin added that caffeine did not significantly affect one crucial measure of fetal development — abdominal circumference.

“Traditionally when looking at fetal growth, abdominal circumference is probably the most important feature of predicting which fetuses are larger and which fetuses are smaller,” Berkin said. “In the calculations that we use to determine fetal growth, abdominal circumference is weighed heavier than all the other parameters.”

There are several theoretical reasons to suspect that caffeine could inhibit fetal growth, Gleason said.

“We know that caffeine and its primary metabolite paraxanthine both cross the placenta, but the fetus lacks the enzymes to break down or clear caffeine from its system,” Gleason said. As caffeine builds up in fetal tissues, it could disrupt growth in the womb.