The fetal acoustic stimulation test: a reliable and cost effective method of antepartum fetal monitoring.

ABSTRACT

RATIONALE A healthy foetus, if exposed to external sound stimulation, often responds with vigorous movements which can be felt by the mother. A hypoxic foetus usually does not show this response. OBJECTIVES: To evaluate the fetal acoustic stimulation test (FAST) in antepartum fetal monitoring. DESIGN AND SETTING: Prospective interventional study carried out at the Teaching Hospital, Galle. METHOD: An initial non-stress test (NST) followed by a FAST using the Corometric model 146 was carried out in 423 high risk pregnant women. The response to FAST was assessed and compared with the NST. A repeat NST was recorded in women who had an initial non-reactive NST. The results of the NST and FAST were compared with the fetal outcome if the woman delivered within 24 hours. OUTCOME MEASURES: Maternal perception of fetal movements after FAST, results of NST before and after FAST, and the babies' 5 minute Apgar scores if delivered within 24 hours of the FAST. RESULTS: Of the women, 349 (82.5%) noticed fetal movements after FAST. Of the 167 women who complained of absent or reduced fetal movements, 67% felt fetal movements after FAST. Ninety one had a nonreactive NST and 43 (47%) became reactive after FAST. Compared to the NST, the FAST had less sensitivity (93% vs 100%, p = 0.01), better specificity (79% vs 45%, p = 0.001), better positive predictive value (67% vs 50%, p = 0.02), similar negative predictive values (96% vs 100%, p > 0.05) and better accuracy (83% vs 69%, p = 0.03) in predicting neonatal asphyxia (5 minute Apgar score < 7) if the baby was delivered within 24 hours after the test. CONCLUSION: The FAST is a reliable, cost effective screening test for antepartum fetal monitoring. It significantly reduces the false positive (non-reactive) NST and has a good negative predictive value.