Newborn infants' cry after heel-prick: analysis with sound spectrogram

The aim of the study was to test the hypothesis that a newborn infant's cry can be used in conjunction with an instrument to measure pain. Crying due to pain was analysed after a heel-prick stimulus. In a prospective, descriptive study, 50 healthy newborn infants were subjected to a heel-prick for phenylketonuria screening. Their cries of pain were recorded and analysed. Duration of the crying sound was analysed and, using a sound spectrogram, the fundamental frequency and the cry melody of the first five cry sounds were analysed. The analysis showed that the crying sound after the painful stimulus of the heel-prick had a significantly higher fundamental frequency and lasted longer at the first than at the fifth cry. The first cry had a more varied crying melody than the fifth. There were large differences between individual cries from a single infant, as well as in the duration of each cry, total crying time, and fundamental frequencies between infants. While the first cry was more like a cry of pain, the fifth cry more resembled crying for reasons other than pain. The results suggest that newborn infants react to pain in a recognizable way. However, other stimuli may cause a similar reaction. Crying can therefore be used to measure pain in newborn infants only when the cause of crying is known.