Introduction: The aim of our study is to determine prevalence and clinical significance of the presence of amniotic fluid sludge among asymptomatic patients at high-risk for spontaneous preterm delivery, prospectively.Material and methods: In our study, 99 patients at high risk for spontaneous preterm delivery were evaluated for the presence of amniotic fluid sludge with transvaginal ultrasonography at 20-22, 26-28, and 32-34 gestational weeks, prospectively; between August 2009 and October 2010 in Hacettepe University Hospital. And, these patients were followed up for their delivery weeks and pregnancy outcomes. We defined the high-risk group as the patients possessing one or more of the followings; a history of spontaneous preterm delivery, recent urinary tract infections, polyhydramnios, uterine leiomyomas, mullerian duct anomalies, and history of cone biyopsy or LEEP. Patients with multiple gestations, placenta previa, fetal anomalies, or symptoms of preterm labor at first examination were excluded.We have obtained ethical board approval from Hacettepe University (16.07.2009-HEK/No:09-141-59).Results: The prevalence of amniotic fluid sludge in the study population was 19,6% (18/92). The rates of spontaneous preterm delivery at <37 weeks of gestation were 66,7% (12/18), within the patients with sludge and 27,0% (20/74) within the patients without sludge. Patients with sludge had a higher rate of spontaneous preterm delivery (p=0.002). A higher proportion of neonates born to patients with amniotic fluid sludge had a neonatal morbidity (50% (9/18) vs. 24,3% (18/74), p=0.044) and died in the perinatal period, (p=0,013) than those born to patients without sludge. When we combined sludge and cervical lenght (CL) (<25mm) and used it as a screening test to identify women at risk for preterm delivery; it catched more women with preterm delivery, (p=0.000). While sensitivity of sludge was 37,5%, and sensitivity of CL was 34%, sensitivity of sludge positive or CL25mm was 56% for preterm birth (PTB) in high-risk group.Conclusions: The prevalence of amniotic fluid sludge is 19,6% and sludge is an independent risk factor for spontaneous preterm delivery among asymptomatic patients at high-risk for spontaneous preterm delivery. PTB is by far the leading cause of infant mortality, and prevention of PTB has been an elusive goal. When sludge added to screening, we can catch more PTB.