Pregnant women who drink six cups of coffee every day may have smaller babies than women who consume less caffeine.
Researchers from the Erasmus Medical Centre in Rotterdam followed more than 7,300 Dutch women from early pregnancy onward of whom between 2 and 3% said they consumed the caffeine equivalent of six cups of coffee per day during any trimester.
On average, their babies' length at birth was slightly shorter than that of newborns whose mothers had consumed less caffeine during pregnancy.
"Caffeine intake seems to affect length growth of the foetus from the first trimester onwards," researcher Rachel Bakker told Reuters Health.
Heavy caffeine consumers also had an increased risk of having a baby who was small for gestational age - smaller than the norm for the baby's sex and the week of pregnancy during which he or she was born.
That finding, however, was based on a small number of babies, and the significance is uncertain. Of 104 infants born to women with the highest caffeine intakes, seven were small for gestational age.
The findings, reported in the American Journal of Clinical Nutrition
add to the conflicting body of research into whether caffeine during pregnancy affects foetal growth.
Some studies, for instance, have linked regular caffeine consumption during pregnancy - even a relatively modest one or two cups of coffee a day - to an increased risk of low birth weight.
But other studies have found no such effects.
Researchers have also come to conflicting conclusions as to whether caffeine affects the risk of miscarriage. In this latest study, Rachel Bakker and colleagues at the Erasmus Medical Centre used ultrasound scans to monitor foetal growth over the course of pregnancy in 7,346 women.
At each trimester, the women reported on their usual intake of coffee
and tea. Most women consumed less than the equivalent of four cups of coffee per day at any point in pregnancy, but between 2 and 3% downed six or more cups' worth of caffeine.
Overall, babies born to heavy caffeine consumers were slightly shorter, on average, at birth and during all three trimesters of foetal development, based on the ultrasound tests.
Bakker said the implication is that pregnant women should not consume more than six cups of coffee per day. However, the findings also do not mean that less coffee is generally "safe" during pregnancy.
"We only studied the effect of caffeine on foetal growth," Bakker said. "Future studies on possible other effects of maternal caffeine intake are therefore needed."Did you drink a lot coffee during your pregnancy?