There are conflicting reports about the association of maternal caffeine intake with adverse pregnancy outcomes, specifically fetal growth restriction (FGR). Differences in study design and exposure definition have been partly responsible. In order to study the association between maternal caffeine consumption and FGR, I prospectively quantified caffeine intake in pregnancy from all known sources, assessed caffeine metabolism in pregnancy and used serial ultrasound growth scans to identify FGR fetus.\ud In a prospective study of 1340 pregnant women in Leicestershire, UK, I quantified total caffeine intake from 4-weeks prior and throughout pregnancy using a validated caffeine assessment tool. Caffeine half-life (used here as proxy for clearance) was determined by measuring caffeine in saliva after a caffeine challenge. The primary outcome measure was FGR, which was determined by customised birth weight centile calculator and in a subgroup by serial ultrasound growth scan.\ud Mean caffeine consumption decreased in the 1st and then increased in the 3rd trimester. Caffeine consumption throughout pregnancy was associated with an increased risk of FGR: OR = 1.2 (95% CI, 0.9 to 1.6) for 100-199 mg/day, OR = 1.5 (1.1 to 2.1) for 200- 299 mg/day, and OR = 1.4 (1.0 to 2.0) for over 300 mg/day compared to < 100 mg/day (Ptrend< 0.001). There was some evidence that the effect of caffeine on FGR was strongest in women with faster caffeine clearance (P = 0.06). On comparing outcome measure of FGR as defined by serial ultrasound growth scans in pregnancy and customised centile calculator, there was a moderate degree of agreement between the two methods (κ = 0.38, CI 0.26, 0.49).\ud Caffeine consumption during pregnancy is associated with an increased risk of FGR and this effect is continuous throughout pregnancy. This effect was observed from four weeks before pregnancy and was statistically significant in the first trimester. A public health recommendation for pregnant women would be to reduce or limit the caffeine intake to a maximum of 2 cups of tea/coffee per day. Future research is required to study the mechanistic effect of caffeine on trophoblastic tissue
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