Caffeine, which readily crosses the placenta, increases circulating catecholamine levels and may cause uteroplacental vasoconstriction and fetal hypoxia. Some but not all studies have found an association between high caffeine intake and low birth weight or the risk of a small-for-gestational age infant. This randomized double-blind study was carried out in 1207 Danish women who, while pregnant, regularly drank 3 or more cups of coffee a day. All were entered into the trial before 20 weeks’ gestation. None of the women had previously had a low-birth-weight infant or preterm delivery. They were asked to replace their usual coffee with either caffeinated (n = 568) or decaffeinated (n = 629) instant coffee, and were interviewed at gestational weeks 20, 25, and 34, and again 4 weeks after the expected date of delivery. Birth weight data were available for 1150 live-born singleton infants, and data on gestational age for 1153 infants.
The difference in caffeine intake between the 2 study groups averaged 182 mg daily. Neither birth weight nor mean length of gestation differed significantly between women drinking caffeinated coffee and those assigned to decaffeinated coffee. After adjusting for gestational length, parity, prepregnancy body mass index, and smoking status at entry to the trial, infants whose mothers drank caffeinated coffee had a mean birth weight 16 g higher than those in the decaffeinated group. There was no significant between-group difference in gestational age. The level of coffee consumption at the outset did not influence the findings. Women who smoked more than 10 cigarettes a day at the outset had smaller infants if drinking caffeinated rather than decaffeinated coffee, the mean difference being 263 g. There were no significant group differences in the incidence of preterm birth, small-for-gestational age infants, or 5-minute Apgar scores less than 7.
No evidence was found in this randomized double-blind study that moderately reducing caffeine intake in the second half of pregnancy has any substantial effect on the length of gestation or on birth weight. It remains possible that, if caffeine affects birth weight, it does so in ways that operate only early in pregnancy. In addition, the possibility that ingredients of coffee other than caffeine may influence birth weight has not been excluded.