Objective: To determine the association of amniotic fluid sludge with impending preterm delivery in women in preterm labor with intact membranes. Methods: This was a retrospective cohort study in women at 22-34 weeks' gestation in preterm labor with intact fetal membranes between February 2008 and May 2009. 16 women with amniotic sludge were identified. Each case of amniotic sludge was matched with 3 randomly selected controls. The primary outcome was delivery within 48 h of examination. Results: Mean cervical length was shorter in women with sludge than women without sludge (20.5 ± 9.0 vs. 27.9 ± 6.9 mm, p = 0.0016). There was a significant difference in the mean ultrasound-to-delivery interval in women with and without sludge (21.7 vs. 49.4 days, p = 0.006). Logistic regression analyses demonstrated that amniotic fluid sludge is a strong independent predictor of impending preterm delivery within 48 h (OR 11.2, 95% CI 1.2-125.9), 7 days (OR 3.8, 95% CI 1.2-17.3) and 14 days (OR 7.5, 95% CI 1.8-30.4). Conclusions: Amniotic fluid sludge on ultrasound scan is associated with impending preterm delivery in women in preterm labor with intact membranes. Copyright © 2011 S. Karger AG, Basel.