Moderately reducing caffeine intake for the second half of pregnancy does not seem to affect birthweight or length of gestation, according to the results of a randomized, double-blind, placebo-controlled study involving about 1,200 women from Denmark. 

The women, who were 20 weeks' pregnant or less and reported drinking at least 3 cups of caffeinated coffee a day prior to entering the study, were divided into two groups. One group drank caffeinated instant coffee, while the other drank decaffeinated instant coffee for the remainder of their pregnancies. The women drinking decaf coffee had a mean caffeine intake that was 182 mg lower than that of the women in the caffeinated group.  

After adjusting for a number of variables, the researchers calculated that the mean birthweight of the babies born to the decaf-drinking mothers was a clinically insignificant 16 g higher (95% CI; -40 to  73, 

=0.57) than that of the babies born to the caffeine-drinking moms. No significant difference emerged regarding gestational age either. 

Bech BH, Obel C, Brink T, et al. Effect of reducing caffeine intake on birth weight and length of gestation: randomized controlled trial. 