A 34-year-old woman, 28 weeks pregnant, presents with a 3-day history of dysuria, urinary frequency, and suprapubic pain. She has no fever or loin pain. A mid-stream urine (MSU) sample shows >10^5 cfu/mL of Escherichia coli, sensitive to nitrofurantoin and amoxicillin. What is the most appropriate antibiotic treatment?