A randomised comparison between amniotic fluid index and maximum pool depth in the monitoring of post-term pregnancy.

OBJECTIVE: To compare the impact of two different ultrasound methods for assessing amniotic fluid volume on the incidence of obstetric interventions in post-term pregnancies.

DESIGN: A prospective randomised controlled trial.

SETTING: Liverpool Women's Hospital.

PARTICIPANTS: Five hundred women with singleton, uncomplicated pregnancies with gestational age > or = 290 days.

INTERVENTIONS: Random allocation to fetal monitoring by either: 1. amniotic fluid index and computerised cardiotocography, or 2. maximum pool depth and computerised cardiotocography.

MAIN OUTCOME MEASURES: Primary: caesarean section. Secondary: the number of abnormal monitoring tests, induction of labour, intrapartum management and neonatal outcome.

RESULTS: The number of abnormal amniotic fluid indices was significantly higher than the number of abnormal maximum pool depths (10% vs 2.4%; OR 4.51, 95% CI 1.82-11.21; P = 0.0008) which resulted in more inductions for abnormal post-term monitoring in the amniotic fluid index group (14.8% vs 8.4%; OR 1.89; 95% CI 1.07-3.33; P = 0.0362) and more intrapartum electronic fetal monitoring (94.4% vs 88.4%; OR 2.21; 95% CI 1.13-4.29; P = 0.0255). There were no other statistically significant differences in outcomes related to labour and delivery, but there was a trend towards more caesarean sections in the amniotic fluid index group (18.8% vs 13.2%), in particular caesarean sections for fetal distress (8% vs 4%). There were no perinatal deaths and no statistically significant differences in perinatal outcome between the two groups.

CONCLUSIONS: Published reference ranges for amniotic fluid index overestimate the number of abnormal results in post-term pregnancies. Their use, when compared with maximum pool depth, is likely to increase the number of obstetric interventions with, as yet, an uncertain impact on perinatal mortality and morbidity. It is possible that antepartum fetal assessment in pregnancies where the risk of adverse perinatal outcome is very low may cause, rather than prevent morbidity.