Caffeine consumption during pregnancy may be associated with an increased risk of fetal growth restriction, a study has found.

Caffeine consumption was assessed in a prospective longitudinal observational study in around 2,500 women recruited early in pregnancy. Each participant was tested and categorised as a fast or slow metaboliser.

Mean caffeine intake during pregnancy was 159 mg/day. Around 62% of the cohort's caffeine intake came from tea. Coffee, cola drinks, chocolate and soft drinks accounted for 14%, 12%, 8% and 2% respectively.

One mug of ordinary coffee contains 100mg caffeine, the concentration is higher for filter coffee. Tea contains under 40mg per mug and cola drinks around 8mg per can. 'Energy' drinks have around 40mg/can.

Outcomes were collected from delivery statistics. Low birthweight was defined as <10th centile on customised centile charts.

A statistically significant trend towards low birthweight was seen with increasing levels of caffeine intake (test for trend p= 0.004). Compared with caffeine intake of <100mg/day, the OR for having a low birthweight baby increased to 1.2 (95% CI 0.9 -1.6) for 100-199mg/day, to 1.5 (1.1-2.1) for 200-299mg/day, and to 1.4 (1.0-2.0) for >300mg/day.

The results were controlled for alcohol and smoking, both variables which are linked to caffeine intake.

Fast metabolisers were found to have a higher risk of fetal growth restriction than slow metabolisers.

Caffeine crosses the placenta but neither the placenta nor the fetus has the ability to metabolise it.

The Food Standards Authority (FSA) has updated its nutritional advice to pregnant women based on this study and now recommends no more than 200mg should be consumed per day by women during pregnancy.¹

The evidence from this study is suggestive but not proof that caffeine causes fetal growth restriction and there has been some dispute about its conclusions in the BMJ's correspondence pages. The FSA advice however seems prudent.

CARE Study Group. Maternal caffeine intake during pregnancy and risk of fetal growth restriction: a large prospective observational study. BMJ 2008;337:a2332 doi:10.1136/bmj.a2332