When treating urinary tract infection (UTI) in the third trimester, the antibiotic of choice should be

Although quite effective, sulfonamides should be avoided during the last few weeks of pregnancy because they competitively inhibit the binding of bilirubin to albumin, which increases the risk of neonatal hyperbilirubinemia. Nitrofurantoin may not be tolerated in pregnancy because of the effect of nausea. It should also be avoided in late pregnancy because of the risk of hemolysis due to deficiency of erythrocyte phosphate dehydrogenase in the newborn. Tetracyclines are contraindicated during pregnancy because of dental staining in the fetus. Thus, the drugs of choice for treatment of UTI in pregnancy are ampicillin and the cephalosporins.