While observation of fetal breathing movements has been used in fetal assessment, quantitative parameters (percent time spent in breathing [incidence], breath rate, or breath rate variability) have not been adequately evaluated as predictive tools. We examined 283 patients with high-risk pregnancies between 32 and 42 weeks' gestation and correlated their fetal breathing movement parameters with the rates of perinatal mortality, intrapartum fetal distress, neonatal acidosis, low 5-minute Apgar score, and intrauterine growth retardation. Fetal breathing data from standardized 60-minute biophysical tests were analyzed and compared with our institutional standards. Parameter values greater than 2 SD from the means of a previously studied normal population were considered abnormal. Whereas no individual parameter was a highly accurate predictor of adverse outcome, a fetal breathing movement incidence of less than 5% provided the best cutoff for diagnostic accuracy. Seventy percent of fetuses with 30 minutes of apnea had normal outcomes, whereas abnormally high breath rates (greater than 60 breaths/min) and low breath rates (less than 33 breaths/min) occurred with equal frequency among normal and pathologic fetuses. Breath interval variability was of no benefit in detecting fetuses with poor outcomes. Observation of fetal breathing movement incidence appeared to be most effective in pregnancies complicated by chronic hypertension and least effective in those with preeclampsia.