This patient is pregnant and has asymptomatic bacteriuria caused by Escherichia coli susceptible to multiple antibiotics, so she should begin a course of amoxicillin. Asymptomatic bacteriuria during pregnancy increases the risk of pyelonephritis and has been associated with preterm birth and low-birthweight infants. Therefore, pregnancy is one of the few indications for screening for bacteriuria in asymptomatic patients and should occur in the first and third trimesters. Bacteriuria is defined as bacterial counts of 10⁵ or greater colony-forming units/mL on urine culture; the prevalence of asymptomatic bacteriuria in pregnancy is approximately 4% to 7%. Risk factors for bacteriuria during pregnancy include lower socioeconomic status, increased parity, older age, increased sexual activity, diabetes mellitus, sickle cell trait, and history of urinary tract infection. In patients with untreated bacteriuria early in pregnancy, approximately 20% to 40% will develop acute symptomatic pyelonephritis later in pregnancy. Antibiotic therapy should be culture guided using an antibiotic with a known safety profile in pregnancy, such as amoxicillin. Amoxicillin-clavulanate and cephalexin are also effective and safe for many cases of asymptomatic bacteriuria. Urine cultures should be obtained 1 to 2 weeks after completing therapy and monthly for the remainder of the pregnancy.

Nitrofurantoin is classified by the FDA as pregnancy category B based on a long history of safe and effective use without significant teratogenic potential in available studies. Because of this, the American College of Obstetricians and Gynecologists Committee Opinion on sulfonamides, nitrofurantoin, and risk of birth defects state that if other treatment options cannot be used, then nitrofurantoin may be used as a first-line agent for the treatment of UTI during the second and third trimesters and in the first trimester if no suitable alternative antibiotics are available. However, a large population-based cohort study from Norway published in 2013 analyzed data from more than 180,000 births to estimate whether exposure to nitrofurantoin is associated with increased incidence of negative pregnancy outcomes. Neonates exposed to nitrofurantoin in the last 30 days before delivery had no increase in birth defects but did have a significantly higher rate of neonatal jaundice requiring treatment than neonates exposed to a beta-lactam antibiotic during the same stage of pregnancy (10.8% and 8.8%, respectively; P = 0.023). Therefore, although not absolutely contraindicated, these data suggest that the use of nitrofurantoin should be avoided in the last 30 days of pregnancy provided that a suitable, safe, antimicrobial agent is available, such as with this patient.

Trimethoprim-sulfamethoxazole is also an effective agent for uncomplicated cystitis when the rate of resistance of E. coli in the community is less than 20%. However, trimethoprim-sulfamethoxazole should not be used in pregnant patients near term because it may cause hyperbilirubinemia and kernicterus in the newborn.

No treatment is necessary for asymptomatic bacteriuria except during pregnancy or if a patient is scheduled to undergo an invasive urologic procedure, which is the only other indication for screening asymptomatic persons. Because this patient is pregnant, treatment for identified bacteruria is indicated.