Computerised antenatal fetal heart rate recordings between 24 and 28 weeks of gestation

Objective To assess computerised fetal heart rate recordings between 24 and 28 weeks of gestation for gestation related differences.
Design Prospective, cross sectional observational study.
Setting Liverpool Women's Hospital.
Population 112 women: 28 at 27 weeks, 30 at 26 weeks, 27 at 25 weeks and 27 at 24 weeks of gestation, respectively.
Methods Fetal heart recordings of 60 minutes duration were performed once in each pregnancy using the System 8000 fetal heart rate programme (System 8000, Oxford Sonicaid Ltd, Chichester, UK). For each gestational age, records were analysed for short term variation, basal heart rate, accelerations and time spent in high episodes.
Results The mean short term variation increased with gestation (   P =0.05  ). No record had a short term variation <4 msecs. There was no relationship between heart rate and increasing gestation. The mean number of accelerations per record increased with increasing gestation (   P <0.01  ). 20% of recordings showed no accelerations  >15 bpm  . The mean duration spent in episodes of high variation increased with gestation (   P =0.05  ). 13% of recordings showed no time spent in high episodes. All fetuses had normal outcomes at delivery.
Conclusions Absence of episodes of high variation or absence of accelerations is not an abnormal finding at lower gestations. The standard threshold of 4 msecs for short term variation appears to remain valid at lower gestations. These differences should be considered when using computerised CTG analysis at early gestations.