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 non-reactive 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.