HOW FREQUENCY SHOULD THE AMNIOTIC-FLUID INDEX BE PERFORMED DURING THECOURSE OF ANTEPARTUM TESTING

OBJECTIVE: Our purpose was to evaluate the need for frequent amniotic fluic volume assessments in our antepartum testing unit. STUDY DESIGN: In a retrospective analysis of data accumulated over 1 year in our an tepartum 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 da ys. The results were stratified on the basis of the subsequent amnioti c fluid index value and estimated gestational age. Comparisons were ma de relating to amniotic fluid index testing intervals of 4 versus 7 da ys. RESULTS: Patients with an amniotic fluid index >8 cm had a 2.3% ch ance (128/5677) of having oligohydramnios (amniotic fluid index less t han or equal to 5.0 cm) in the next 4 days. Those patients with low-no rmal amniotic fluid index values (5 to 8 cm) had a 16.2% chance (96/59 3) of having oligohydramnios in the next 4 days. There were few patien ts with an amniotic fluid index less than or equal to 5 cm who had sub sequent amniotic fluid index values measured. Patients with an amnioti c fluid index >8 cm had a 2.2% chance (156/6946) of having oligohydram nios within 7 days, and those with low-normal amniotic fluid index val ues had a 16.3% chance (105/643) of having oligohydramnios within 7 da ys. Stratifying the results by gestational age reveals that patients a t greater than or equal to 41 weeks of gestation had a 23.3% chance of having oligohydramnios within 4 days if the current amniotic fluid in dex was found to be 5 to 8 cm and a 7.4% chance if the current amnioti c 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 patient s at <41 weeks of gestation undergoing antepartum testing, weekly asse ssments of amniotic fluid index is probably adequate if the initial me asurement is in the normal range (greater than or equal to 8 cm) becau se 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;he basis of a higher risk for an am niotic fluid index less than or equal to 5 cm within 4 days (12.3%). F or all patients at greater than or equal to 41 weeks of gestation, twi ce-weekly amniotic fluid index assessments are recommended regardless of the initial measurement.