It's considered a pregnancy tonic, and it can be safely drunk several cups daily. Dandelion tea is also very helpful for water retention during pregnancy, and since it has natural potassium, it helps prevent becoming depleted (over the counter & prescription diuretics tend to flush everything and thus you become low on potassium, sodium, etc.).
EDIT:
Almost forgot: Both are gentle diuretics, so you may need to drink 2-4 cups a day to really see results. Drinking these types of teas regularly will help prevent that overload swelling, but it may take larger doses, resting with feet up, drinking extra water, etc., to really get swelling down if it's already fairly bad.
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