I've been looking for info on Yerba Mate while pregnant (am currently ttc). A lot of websites have warnings not to drink it while pregnant or bf. But it is unclear if they have a reason for the warning. They could be advising on the side of caution, because they don't know if there'd be any problems.
I'm going to stick to organic green tea, to be safe. Looks like green tea has many of the same antioxidants and other helpful stuff as mate.