Background: asymptomatic bacteriuria (ASB) during pregnancy is the primary cause of acute pyelonephritis, preterm labor, low birth weight fetus, etc., if left untreated. Adequate and early treatment reduces the prevalence of these obstetric complications. Objectives: this study aimed to determine the prevalence of ASB during pregnancy and determine the association between asymptomatic bacteriuria and spontaneous preterm birth. Patients Methods: this was a case control study of 120 preterm pregnancies and 40 term pregnancies among those attending El-Galaa Teaching Hospital, Obstetric Emergency Unit to evaluate the incidence of ASB and determine the association between asymptomatic bacteriuria and spontaneous preterm birth from March 2016 to March2017.Urine analysis and culture with antibiotic sensitivity were performed . Results: out of the 160 patients studied, ASB was observed in 7 patients (6 cases in preterm group (5%) and 1 case in full term group (2.5%) with no significant difference between the two groups (p-value˃ 0.05), the incidence of ASB in preterm labor was quite lower 5%. E. coli was the most common pathogen (55.14%) followed by proteus 28.57 followed by klebsiella (14.28%). Conclusion: all pregnant women should be screened by urine culture to detect asymptomatic bacteriuria at their first visit to prevent overt urinary tract infections (UTI) and other complications in both mother and fetus.