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The presence or absence of cyclic variation (cyclicity) of fetal heart rate patterns was prospectively investigated in fetuses between 25 and 32 weeks' gestation. All fetuses were delivered by cesarean section before the onset of labor. The relationship between the presence or absence of fetal heart rate cyclicity and fetal acidosis, as determined by cord pH measurements, was investigated. The sensitivity, specificity, positive predictive value, and negative predictive value of this method in predicting fetal acidosis were: 100, 90, 50, and 100%, respectively. All fetuses with reactive nonstress tests also had fetal heart rate cyclicity present and none were acidotic at birth. The majority of fetuses (68.8%) with nonreactive nonstress tests had fetal heart rate cyclicity present and none were acidotic; fetuses with nonreactive nonstress tests and absence of fetal heart rate cyclicity were acidotic in 50% of the cases. These preliminary data suggest that the presence or absence of fetal heart rate cyclicity may help to select the healthy preterm fetuses with nonreactive nonstress testing who are in good condition and therefore in no need for further testing.