Objective: To compare the impact on perinatal outcome of two different protocols for antenatal fetal monitoring after 42 weeks gestation.
Design: A prospective randomised controlled trial.
Setting: Liverpool Maternity Hospital.
Subjects: One hundred and forty-five women with singleton, uncomplicated pregnancies after 42 weeks of gestation.
Interventions: Random allocation to fetal monitoring by either: 1. a modified biophysical profile comprising of computerised cardiotocography, amniotic fluid index, and assessment of fetal breathing, tone and gross body movements; or 2. standard cardiotocography and maximum pool depth.
Outcome measures: Cord pH at delivery, number of abnormal monitoring tests, intrapartum management, mode of delivery and neonatal outcome.
Results: There were significantly more abnormal antenatal monitoring results in the modified biophysical profile group (47.2% vs 20.5%; odds ratio = 3.5, 99% CI = 1.3-9.1). There were no differences in cord blood gases, neonatal outcome, or in outcomes related to labour and delivery between the two groups, but a trend towards more obstetric interventions in the modified biophysical profile group was noted. Amniotic fluid volume after 42 weeks was more likely to be labelled as abnormal with amniotic fluid index than with maximum pool depth (44.4% vs 15.1%; odds ratio = 4.5, 99% CI = 1.6-12.8).
Conclusions: The results suggest that after 42 weeks fetal monitoring with the modified biophysical profile does not improve pregnancy outcome as measured by umbilical cord pH, but is more likely to yield an abnormal result. The higher incidence of abnormal monitoring results in the modified biophysical profile group was likely to be caused by different methods of amniotic fluid assessment in the modified biophysical profile group (amniotic fluid index) and simple monitoring group (maximum pool depth). The use of amniotic fluid index as a test of fetal wellbeing in prolonged pregnancy may lead to more obstetric interventions with, as yet, unclear impact on the perinatal outcome.