metabolizes caffeine more slowly during pregnancy, especially in the last few months; the half-life of caffeine increases from approximately 5 to 18 h during the second and third trimesters. Blood caffeine concentrations are therefore raised during pregnancy with no change in intake. In contrast, smoking is known to increase caffeine metabolism appreciably.

Although very high doses of caffeine are terato-genic in animals, no link between consumption during pregnancy and birth defects has been demonstrated in humans. However, high maternal caffeine intakes (>500mg/day) have been associated with increased fetal heart rate and newborn cardiac arrhythmias. Although there is no reliable evidence linking caffeine intake during pregnancy with sudden infant death syndrome or preterm birth, a number of studies have reported significantly increased risks of spontaneous abortion and low birth weight with caffeine intakes greater than 300mg/day and some have also demonstrated increased risks at lower intakes (150 mg and higher). However, whether these associations are causal remains to be established because the effects of confounding factors such as smoking and alcohol consumption on these outcomes could not be determined. Concern has also been expressed about the reliability of using self-reported caffeine intake in many of these studies.

The lack of consistency between studies, particularly in relation to the dose at which an effect is reported, makes it very difficult to identify a threshold level of caffeine intake that presents an increased risk during pregnancy. The available data suggest that a moderate level of caffeine consumption is safe, although there may be concerns at high levels of intake. The Food Standards Agency in the United Kingdom advises pregnant women to moderate their caffeine intake to no more than 300 mg/day. This is equivalent to approximately three or four cups of coffee (Table 7). This figure is endorsed by the European Union Scientific Committee on

Table 7 Approximate equivalent of 300 mg of caffeine

4 average cups or 3 average-size mugs of instant coffee

3 average cups of brewed coffee 6 average cups of tea

8 cans of regular cola drinks

4 cans of 'energy' drinks

400 g (8 standard 50-g bars) of milk chocolate

Foodstuffs, which states that ''up to 300mg/day appear to be safe.'' In practice, many pregnant women reduce their coffee intake as a result of a spontaneous aversion to the taste and smell, particularly in early pregnancy.

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