I9 weeks preg with a uti. prescribed 3 tablets a day of nitrofurantoin [30 tablets] but instructions say 4 tabs over 7 days? what is the correct dose?

Double check with MD: I would call the office to clarify - usually nitrofurantoin is given 4x a day, but maybe there was some reason they wanted only 3x a day. That might be if you have any pre-existing kidney trouble, in that case - a slightly lower amount might be better for you. This medicine is considered 'safe' in pregnancy - for the stage of pregnancy you are in now. Good to get the infection over and done with.