Antepartum fetal testing. II. The acceleration/constant ratio: a nonstress test.

For a period of one-half hour of undisturbed fetal monitoring, periodic changes of the FHR in response to spontaneous fetal movements (FM's) were recorded. FM's in patients who subsequently had positive OCT's were less likely to show accelerations (p = 0.001) and more likely to show variable decelerations (p = less than 0.001) and no change (p = less than 0.001) in the FHR when compared with patients who did not have a positive OCT. A ratio between the number of FM's associated with accelerations and the sum of FM's associated with no change and decelerations was defined as the acceleration/constant (A/C) ratio. The outcome in patients who exhibited reactive tests (i.e., A/C ratio greater than 1) was more favorable than the outcome in patients with nonreactive tests (i.e., A/C ratio less than or equal to 1). Patients with positive OCT's universally showed nonreactive tests, whereas patients with false positive OCT's were more likely to have reactive tests. Evidence is presented to suggest that the A/C ratio is more predictive of the intrauterine environment than the OCT.