- Objective To test the incidence and sonographic parameters of pyelonephritis during pregnancy, and to examine risk factors and pregnancy outcomes of women with acute antepartum pyelonephritis. Study design A retrospective population-based study comparing all singleton pregnancies of patients with and without acute antepartum pyelonephritis was performed. Patients lacking prenatal care as well as multiple gestations were excluded from the study. Multiple logistic regression models were used to control for confounders. Results Out of 219,612 singleton deliveries in 1988–2010, 165 women (0.07%) suffered from acute antepartum pyelonephritis. Abnormal sonographic findings were found in 85.7% of the patients with pyelonephritis. Pyelonephritis was significantly associated with nulliparity (46.1% vs. 24.4%, p < 0.001), younger maternal age (26.3 ± 6.0 vs. 28.6 ± 5.8 years, p < 0.001), intrauterine growth restriction (IUGR) (6.7% vs. 2.1%, p < 0.001), placental abruption (3.6% vs. 0.7%, p < 0.001), low 1 min Apgar scores (10.3% vs. 6.0%, p < 0.05), urinary tract infection (UTI) (4.2% vs. 0.4%, p < 0.001) and preterm delivery (less than 37 weeks gestation; 20.0% vs. 7.8%; p < 0.001). Using a multivariable analysis, independent risk factors for acute antepartum pyelonephritis were nulliparity (OR 2.0; 95% C.I 1.4–2.9; p < 0.001), UTI (OR 10.3; 95% C.I 4.8–22.1; p < 0.001) and younger maternal age (OR 0.96; 95% C.I 0.93–0.99; p = 0.009). Using another multivariable analysis, with preterm delivery as the outcome variable, acute antepartum pyelonephritis was found as an independent risk factor for preterm delivery (OR 2.6; 95% C.I 1.7–3.9; p < 0.001). Conclusion Acute antepartum pyelonephritis is associated with adverse perinatal outcomes and specifically is an independent risk factor for preterm delivery.