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Ingesting more daily caffeine than is present in one cup of coffee while pregnant is associated with reduced fetal growth according to a well-designed study from a major medical journal.

Caffeine is easily absorbed and passes 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. It makes logical sense that 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 that caffeine levels can vary substantially among even when they ingest the same amount of caffeine.

An excellent article published in one of the world's leading medical journals, the British Medical Journal (BMJ), has provided substantial insight into this question by eliminating many of the problems with earlier studies. They 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. They 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.

Although major medical organizations currently set the recommended limit for caffeine at about two to three time that amount (see link below), this study provides strong evidence that these guidelines need to be re-evaluated.

My current advice to a java-loving pregnant woman? If she absolutely can't or won't discontinue drinking caffeine-containing beverages during pregnancy -- which would be the ideal solution given the conflicting evidence about whether any amount of caffeine can be safely ingested during pregnancy -- she should limit her intake to no more than one cup of coffee or two cups of tea per day.

More details about the BMJ study

Current more liberal standards (.pdf file)