A large study has found that caffeine intake is associated with decreased birth weight, and that coffee consumption was associated with marginally increased gestational length.
The research was based on the Norwegian Mother and Child Cohort Study and included more than 59000 women. The main caffeine source was coffee, but tea and chocolate were the main sources in women with low caffeine intake.
Even caffeine consumption below the currently recommended maximum (200mg/day) was associated with an increased risk of the fetus being small for gestational age. Caffeine passes the placental barrier; the fetus does not express the main enzymes that inactivate it, and caffeine metabolites have been found to accumulate in the fetal brain.