Objective What are pregnancy outcomes following screening for asymptomatic bacteriuria, comparing significant growth, mixed growth and non-significant growth against no growth? Design Retrospective cohort study of pregnant women booking for birth. Setting Tertiary UK hospital Population or Sample All patients screened for bacteriuria before twenty weeks’ gestation for whom pregnancy outcome data was available. Methods We used routinely collected data obtained using an automated extract from the hospital electronic patient records database (EPIC, EPIC Systems Corporation) incorporating all patient data including obstetric, neonatal, microbiology and pathology results. Main Outcome Measures Primary outcomes were preterm birth before 37 weeks and pyelonephritis, with pre-planned secondary outcomes including antimicrobial resistance and subgroup analyses. Results 6095 pregnant women met inclusion criteria (6095/10678, 57.1% of all women booked for care in that period). Preterm birth (<37 weeks’ gestation) was more common in women with non-significant growth (<10 5 colony forming units per ml (CFU/ml), 203/2942, 6.9%, p<0.001) and significant growth (≥10 5 CFU/ml, 41/604, 6.8%, p<0.05) compared with no growth (95/2116, 4.5%) on booking mid-stream urine sample. Following adjustment for risk factors, preterm birth remained more common in those with non-significant growth (OR 1.91, 95%CI 1.13-3.26). Pyelonephritis occurred in 5/6095 (0.1%), more commonly in significant growth (3/604, 0.5%, p<0.05). Conclusions Bacteriuria of any kind was associated with subsequent preterm birth in this large single centre cohort study of booking mid-stream urine culture. Pyelonephritis, although rare, was more common with significant growth, with no cases occurring following no growth.