It is unusual for a culture report to be negative when your microscopy report shows so many pus cells. The reverse happens mostly. Anyway, given the situation, cephalosporins are safer in pregnancy
than the quinolones used to treat urinary infections and since your condition warrants treatment, you may take it. Also take lot of fluids to wash away the excess drug from your system. Any drug that is used in pregnancy is weighed against the benefits that it provides. Please follow your doctor's advice. Good luck.