Does Furadantin work for UTIs?

I am 23 weeks pregnant and I've had a UTI for a little over 3 weeks now. I JUST got my medicine Thursday evening (2 days ago) and start taking it Friday morning. The doseage is 25 MG. Nothing has seemed to to change and I still get occasional pain on my side. I'm suppose to take this for a week, twice daily (20 ml/4tsp each time). Does it take awhile for this medicine to work or should it already be working by now? And MOST of all, is this medicine safe for my baby? The doctor and pharmacist said it is safe for the baby, but I'd just like to be for sure.