BACKGROUND Urinary tract infection is a common pregnancy complication. Asymptomatic bacteriuria (AB) can trigger the development of serious complications affecting both the mother and the fetus. OBJECTIVE Determine the frequency of AB among pregnant women attending to antenatal care at the family medicine clinic number 1 of the Instituto Mexicano del Seguro Social, located in Ciudad Obregón, Sonora. METHODS A longitudinal study was carried out from September to December 2004. Seventy-two 72 pregnant women with gestational age of 24 weeks or less were followed up during four months. All pregnant women were selected by a non-probabilistic method. Every patient had a monthly urine culture during the follow up period. RESULTS Among the 72 pregnant women, 16.7% developed symptomatic urinary infections during the follow-up and 25% had at least one positive urine culture without urinary symptoms, being classified as AB, thus receiving treatment. Frequency of positive urine cultures was common at first and fourth months of follow-up. CONCLUSIONS Urine culture is an important component of prenatal care, and helps in identifying a significant number of urinary tract infections that would go otherwise undetected.