- Objective: To examine the significance of abnormal fetal heart rate (FHR) patterns during the second stage of labor in terms of pregnancy outcome. Methods: A prospective observational study comparing women who had abnormal FHR patterns during the second stage of labor with women who demonstrated normal FHR patterns. Results: Abnormal second-stage FHR patterns were found in 420 tracings (75%), whereas 140 tracings (25%) were normal. In a multivariable analysis, nulliparity (odds ratio [OR] 2.5; 95% confidence interval [CI] 1.5, 4.2), cord problems (OR 1.8; 95% CI 1.03, 3.3), and male sex (OR 1.5; 95% CI 1.01, 2.2) were independent factors affecting the occurrence of abnormal second-stage FHR patterns. Patients with abnormal tracings had significantly higher rates of operative delivery compared with patients with normal tracings. The newborns from the case group had significantly higher percentages of Apgar scores lower than 7 at 1 minute, arterial pH lower than 7.2, and base deficit of 12 mmol/L or higher, and six were admitted to the intensive care unit (ICU). A multiple logistic regression model found second-stage late decelerations, bradycardia less than 70 beats per minute, and the presence of abnormal FHR patterns during the first stage of labor to be independently associated with fetal acidosis (determined by pH less than 7.2 and base deficit greater than 12 mmol/L). Conclusion: Late decelerations, bradycardia less than 70 beats per minute, and abnormal FHR patterns during the first stage of labor might jeopardize fetal well-being, and expedited delivery should be considered.