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

OBJECTIVE: Our purpose was to evaluate the need for frequent amniotic fluid volume assessments in our antepartum testing unit. STUDY DESIGN: 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. RESULTS: Patients with an amniotic fluid index >8 cm had a 2.3% chance (128/5677) of having oligohydramnios (amniotic fluid index ≤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 ≤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 within 7 days. Stratifying the results by gestational age reveals that patients at ≥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. CONCLUSIONS: 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 (≥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 ≤5 cm within 4 days (12.3%). For all patients at ≥41 weeks of gestation, twice-weekly amniotic fluid index assessments are recommended regardless of the initial measurement. (AM J OBSTET GYNECOL 1996;174:33-6.)