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Objective: It was our goal to determine the false-negative rate of the biophysical profile, characterize an 18-year variation in the false-negative rate, examine the relationship between the last normal biophysical profile score and death, and compare the false-negative rate of 2 disparate populations.Study Design: Biophysical profile scores of 86,955 patients at 2 medical centers were collected and recorded prospectively. All perinatal deaths occurring within 1 week of a normal score were similarly recorded. The annual false-negative rate, the cumulative false-negative rate, and the ratio of false-negative results in cases of subsequent fetal death to the perinatal mortality rate were calculated.Results: There were 65 fetal deaths among 86,955 fetuses. Over an 18-year study period at one institution, the false-negative rate varied but not significantly. The cumulative false-negative rate was 0.708 per 1000 at one medical center studied and 2.289 per 1000 at the other center. The average interval between last normal score and fetal death was 3.62 days and did not vary significantly between the medical centers.Conclusions: False-negative results in cases of subsequent fetal death reflect events that are subsequent to the last normal test result. Fetomaternal hemorrhage was the single most identifiable fetal cause of false-negative results in cases of subsequent fetal death. The ratio of the false-negative rate in cases of subsequent fetal death to the perinatal mortality rate should be used as a more objective approach to reporting this value, because the false-negative rate likely reflects the underlying perinatal mortality. [ABSTRACT FROM AUTHOR]