The importance of the behavioural state in biophysical assessment of the term human fetus.

We have examined how the different behavioural states exhibited by 78 healthy fetuses at term influence the result of the non-stress test (NST) and biophysical profile score (BPS). In association with state 1F the average recording time to obtain a satisfactory NST and BPS was 27.0 and 26.3 min respectively, and 52.6% and 44.0% respectively failed to achieve a 'normal' result. In contrast, satisfactory results were always obtained in the two active states (2F and 4F), and the average time was 3-5 min. Mature fetuses spend on average one third of the time in state 1F, and knowledge of its characteristics is therefore important, for the interpretation of biophysical tests. Our results suggest that continuation of biophysical recording for at least 40 min is necessary before an unreactive NST or low BPS should be regarded as suspicious. We suggest that interpretation of the NST and BPS should be made in the light of knowledge of normal patterns of behavioural development rather than using an arbitrarily defined scoring system.