Unfortunately there seem to be heaps of drugs in that category of things that have been taken by many pregnant women with no major ill effects but not properly studied. I don't think there's much incentive for drug companies to bother funding research in pregnant women because it would be so hard to recruit people. They all just end up with a 'avoid it if you can but if you really need it then it's probably ok to take, we just won't give you any guarantees' sort of message.