How frequently should the amniotic fluid index be repeated?

Our objective was to determine the most appropriate interval for assessing amniotic fluid volume with amniotic fluid index.

In a retrospective analysis amniotic fluid indexes performed every 3 to 4 days in antepartum testing patients were compared with their follow-up values. Of 10,742 amniotic fluid indexes there were 7393 with follow-up values within 4 days. The results were stratified by current amniotic fluid index, gestational age, and concurrent nonstress test results. The groups were compared with chi 2 analysis.

Patients with normal amniotic fluid index (> or = 8 cm) had a 0.54% chance of oligohydramnios developing in the next 4 days. Those patients with low normal amniotic fluid indexes (5 to 8 cm) had a 5% chance of oligohydramnios developing within the next 4 days, and patients with low amniotic fluid indexes (< or = 5 cm) had a 59% chance of persistent oligohydramnios 4 days after the index examination. Subdividing by gestational age demonstrated that patients > or = 41 weeks' gestation had a 2.6% chance of oligohydramnios developing within 4 days if current amniotic fluid index was between 8 and 15 cm. Results of concurrent fetal heart rate findings did not appear to change the risk for development of oligohydramnios.

In patients < 41 weeks' gestation with normal amniotic fluid indexes, a repeat amniotic fluid index is not necessary for 7 days.