We selected 4012 cCTG records (one trace for each patient) performed in healthy pregnancies from 30th to 42nd gestational week using foetal heart rate (FHR), short-term variability (STV), long-term irregularity (LTI), Delta, approximate entropy (ApEn), spectral components as low frequency (LF), median frequency (MF), high frequency (HF) and LF/(HF + MF) ratio were analysed. Reference nomograms were created and sensitivity and specificity for the prediction of foetal compromise were calculated which were 90% and 89%, respectively. Changes of cCTG parameters according to gestational week were evaluated: FHR (r = -.65) and LF (r = -.87) showed a statistically significant reduction (p < .05) with gestational age. STV (r = .59), LTI (r = .69), Delta (r = .67), and MF (r = .88) showed a statistically significant increase (p < .05) with gestational age. In contrast, for ApEn (r = -.098), HF (r = .14) and LF/(HF + MF) ratio (r = -.47) a non-statistically significant change was found (p > .05). The identification of reference ranges for cCTG indexes in according to gestational age could provide a more objective examination of cCTG trace.
Keywords: Computerised cardiotocography; antepartum foetal monitoring; foetal heart rate; gestational age; reference ranges.