Prospective comparison of the startle test (recoil) and non-stress test

Our purpose was to compare fetal heart rate reaction to external physical stimulation with the non-stress test (NST).

This was a prospective study evaluating documentation of fetal heart accelerations by two methods. The standard NST was performed prior to the ultrasound evaluation. The NST results were not available to the ultrasonographer. M-mode ultrasound was used to establish a stable fetal heart rate. The ultrasound transducer was used to stimulate fetal movement by indentations of the uterus over the fetal small parts. A second fetal heart rate was determined within 15 s after stimulation.

A total of 122 patients had 159 studies performed. The fetal heart rate range due to fetal startle (recoil) was -22 to 14 (median of 3) in the 45 fetuses with non-reactive NSTs and 1 to 38 (median of 15) in the 114 fetuses with a reactive NST (p <0.001). A receiver operating curve comparing the fetal response to the startle and the NST revealed an area under the curve of 0.972, consistent with high specificity and sensitivity.

The fetal heart rate response to external stimulation correlates with the formal NST.