How frequently should the amniotic fluid index be performed during the course of antepartum testing?

Our purpose was to evaluate the need for frequent amniotic fluid volume assessments in our antepartum testing unit.

In a retrospective analysis of data accumulated over 1 year in our antepartum testing unit amniotic fluid index values performed at 3- to 4-day intervals were compared with follow-up values. Of 11,827 amniotic fluid index values, there were 6291 with follow-up values within 4 days. The results were stratified on the basis of the subsequent amniotic fluid index value and estimated gestational age. Comparisons were made relating to amniotic fluid index testing intervals of 4 versus 7 days.

Patients with an amniotic fluid index > 8 cm had a 2.3% chance (128/5677) of having oligohydramnios (amniotic fluid index < or = 5.0 cm) in the next 4 days. Those patients with low-normal amniotic fluid index values (5 to 8 cm) had a 16.2% chance (96/593) of having oligohydramnios in the next 4 days. There were few patients with an amniotic fluid index < or = 5 cm who had subsequent amniotic fluid index values measured. Patients with an amniotic fluid index > 8 cm had a 2.2% chance (156/6946) of having oligohydramnios within 7 days, and those with low-normal amniotic fluid index values had a 16.3% chance (105/643) of having oligohydramnios with 7 days. Stratifying the results by gestational age reveals that patients at > or = 41 weeks of gestation had a 23.3% chance of having oligohydramnios within 4 days if the current amniotic fluid index was found to be 5 to 8 cm and a 7.4% chance if the current amniotic fluid index was found to be normal. Term (estimated gestational age of 37 to 40 weeks) patients had a similar risk of oligohydramnios in 4 days if the amniotic fluid index was low-normal (17.8%) but a risk of 3.6% if the amniotic fluid index was normal.

For patients at < 41 weeks of gestation undergoing antepartum testing, weekly assessments of amniotic fluid index is probably adequate if the initial measurement is in the normal range (> or = 8 cm) because the risk of having oligohydramnios within 7 days is low (2.2%). For patients at < 41 weeks of gestation whose initial amniotic fluid index measurement is in the low-normal range (5 to 8 cm), a scheme of twice-weekly assessment is justified on the basis of a higher risk for an amniotic fluid index < or = 5 cm within 4 days (12.3%). For all patients at > or = 41 weeks of gestation, twice-weekly amniotic fluid index assessments are recommended regardless of the initial measurement.