A prospective investigation was conducted to examine associations between selected conditions encountered during nonstress testing with the intent of explaining false nonreactive results. During a 16-month period, nonstress tests (NSTs) were performed in 315 pregnancies; 64 (12%) of the initial 534 satisfactory tests had nonreactive patterns. Approximately one third (22 of 64, 34%) of these nonreactive tests either remained nonreactive or were followed by a positive or suspicious contraction stress test that same day. When compared with pregnancies that gave reactive test results, a nonreactive pattern was not found to be more commonly associated with maternal obesity, fasting, certain prescribed medications (methyldopa, phenobarbital, insulin), time of day, or recent amniocentesis. Recent cigarette smoking was infrequent (108 tests, 20%) but might explain a falsely nonreactive result. The data suggest that NST results might instead have been influenced by fetal activity and fetal health during the test period.
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