Pregnant women who consume caffeine – whether it’s from coffee or tea – have smaller babies than those who abstain from the stimulant during pregnancy, new research suggests.

Even women who took in less than 200 milligrams of caffeine, the safe cutoff during pregnancy according to the American College of Obstetricians and Gynecologists (ACOG), had a significantly increased risk of delivering prematurely or having a low-birthweight baby.

“Based on the consistent associations we observed, and because many pregnancies are unplanned, we would recommend women who are pregnant or seeking to become pregnant to at least limit their intakes of caffeinated coffee and tea,” lead study author Ling-Wei Chen, a researcher at University College Dublin in Ireland, said in an email.

The study cannot prove cause and effect, he cautioned. But several previous studies have linked caffeine exposure in the womb to negative effects, Chen and his colleagues write in The American Journal of Clinical Nutrition.

In most of those studies, coffee was the main caffeinated beverage consumed.