Providing decaffeinated coffee to women who drank three cups of coffee or more a day in early pregnancy had no effect on birth weight or length of gestation.
We found only small differences in potential confounders at baseline between pregnant women allocated to instant caffeinated coffee and those allocated to instant decaffeinated coffee, and we adjusted for these in analyses.
To ensure good compliance we did not impose a strict protocol on the use of caffeinated beverages during the trial. Still, we obtained a difference in caffeine intake of a magnitude that has previously been reported to have an effect on birth weight.8 The difference in caffeine intake we found (182 mg a day) corresponds to almost three cups of instant coffee a day. We cannot, however, rule out that larger reductions in caffeine may increase birth weight.
Caffeine intake is associated with smoking and alcohol intake, which may influence birth weight. It is possible that a modification of caffeine intake could also influence other lifestyle factors. However, we found no difference between the groups in smoking or alcohol consumption (data not shown).
Women in the decaffeinated group guessed their type of coffee more often than women in the caffeinated group. Women recruited to the study consumed at least three cups of coffee a day, and it is likely that some in the decaffeinated arm had withdrawal symptoms such as headaches.27
Slightly more women were randomised to decaffeinated coffee than to caffeinated coffee because of differences between the groups in requesting additional study coffee. When women requested more coffee the first box from the remaining stack of coffee that matched the first supply was chosen. Since women receiving caffeinated coffee requested additional coffee more often we randomised more women to receive decaffeinated coffee. This modification in randomisation probabilities has an effect only on power and not on internal validity of the study