If positive urine cultures are available, therapy choices should be based on the released sensitivities. 

1st or 2nd trimester:

Trimethoprim is contraindicated in the 1st trimester of pregnancy.

Nitrofurantoin oral 50-100mg 6 hourly for 7 days

or

Nitrofurantoin modified-release (M/R) oral 100mg 12 hourly for 7 days

or

Cefalexin oral 500mg 8 hourly for 7 days

2nd or 3rd trimester:

Nitrofurantoin is contraindicated in the 3rd trimester of pregnancy.

Cefalexin oral 500mg 8 hourly for 7 days

or

Trimethoprim oral 200mg 12 hourly for 7 days

N.B. Trimethoprim is unlicensed for this indication.