Ingesting one cup of coffee each day – and two cups of tea daily – while pregnant is associated with reduced fetal growth according to a well-designed study from the leading British Medical Journal (BMJ). The study followed 2,635 women through their pregnancy, asking them intermittently about their caffeine ingestion, and then testing caffeine levels in their saliva as double checks of the women’s responses.
They also measured the rate with which each woman metabolized Caffeine and Pregnancy. The study found that any ingestion of caffeine above 100 mg/day, the rough equivalent of one cup of coffee or two cups of tea, was clearly associated with a reduction in the rate with which fetuses grew.
Caffeine is easily absorbed and passed freely across the placenta. Ingesting the amount of caffeine found in approximately two cups of coffee is associated with a 25% reduction in blood flow across the placenta. Any substance (or medical condition) that can reduce blood flow to the fetus is a potential cause of reduced fetal weight. A substantial number of scientific studies have examined whether caffeine is associated with “fetal growth retardation.” The results have been contradictory, making recommendations in this area controversial.
There are significant problems with many of these studies making it difficult to determine if their results are valid. A major underlying problem with the vast majority of these studies is that they are retrospective; they ask women to remember how much caffeine they drank during the pregnancy and then look at outcomes. They include no objective evidence that confirm the accuracy of the women’s recollection, and they don’t take into account that there are large variations in how quickly people metabolize caffeine, the effect of which is tha