What is the first-line treatment for a urinary tract infection in a pregnant woman?

The first-line treatment for a urinary tract infection (UTI) in a pregnant woman is nitrofurantoin. This choice is supported by its effectiveness against common pathogens causing UTIs and its safety profile during pregnancy when prescribed in the appropriate trimester. Nitrofurantoin is particularly advantageous because it works specifically in the urinary tract and has a low risk of causing adverse effects to the fetus compared to other options.

Other common antibiotics, such as ciprofloxacin, are generally avoided during pregnancy due to concerns about potential effects on fetal cartilage formation. Trimethoprim-sulfamethoxazole may also be limited in use during the first trimester because of its potential teratogenic effects and the risk of kernicterus later in pregnancy. Amoxicillin, while considered safe in pregnancy, is not typically the first choice due to the increasing resistance patterns observed with Escherichia coli, which is a common UTI pathogen.

In summary, nitrofurantoin is preferred as the first-line treatment due to its efficacy against UTI pathogens, favorable safety profile during pregnancy, and limited risk of fetal harm.