Caffeine has been associated in some animal studies with an increased risk of miscarriage. Studies that reported such correlations were conducted on rats, which have little in common with humans. In most of the studies the amount of caffeine given to the animals was equal to 12 cups of coffee a day. These studies identify 300 mg as the threshold, above which toxicity may occur. One cup of coffee contains 100 mg of caffeine on average, whereas a can of soda contains roughly 50 mg. Therefore, these studies suggest that 1-2 cups of coffee a day can be consumed without risk for miscarriage. In addition, caffeine may exert a beneficial relaxing effect on the uterine muscle through its inhibitory effect on cyclic-AMP phosphodiesterase.