Most physicians will generally recommend that you avoid caffeine as much as possible during your pregnancy. One reason is that caffeine is a known diuretic, which cause you to pass fluids from your body, and can leave you dehydrated. Drinking too much caffeine can also leach calcium from your body, and it is important that you are getting adequate amounts of calcium during pregnancy. Large amounts of caffeine have also been associated with an increased risk for miscarriage, premature birth and low birth weight. If you do want some caffeine during your pregnancy, most practitioners will recommend you limit your intake to less than 300 mg per day.