Oligohydramnios at Term: A Case Report

Maria L. Lanni, CNM, MS; Elizabeth A. Loveless, CNM, MS

Disclosures

J Midwifery Womens Health. 2007;52(1):73-76. 

In This Article

Definition of Oligohydramnios and Amniotic Fluid Index

Oligohydramnios refers to low amniotic fluid. Amniotic fluid can be assessed qualitatively or quantitatively via ultrasound. The quantitative method measures single pockets >2 cms. The most common quantitative measure used in clinical practice is the amniotic fluid index (AFI). The index is calculated by dividing the uterus externally into 4 quadrants and using ultrasound to measure the largest vertical pocket in each quadrant in millimeters, which are then summed to calculate the AFI.[1] An AFI ≤5 cm is the accepted cutoff for the diagnosis of oligohydramnios.[2]

Oligohydramnios occurs in about 1% to 5% of pregnancies at term.[3] It may be associated with uteroplacental insufficiency, congenital anomalies, viral diseases, idiopathic fetal growth restriction (FGR), premature rupture of the fetal membranes, fetal hypoxia, meconium-stained fluid, and/ or postmaturity syndrome.[4] Oligohydramnios may be responsible for malpresentations, umbilical cord compression, and difficult or failed external cephalic version.[5] Oligohydramnios can also be an idiopathic finding in women who have low-risk pregnancies and no medical or fetal complications.[3]

The AFI was introduced in 1987 as a technique to assess the amount of amniotic fluid in the amniotic sac.[3] Although the AFI is widely accepted as the standard to diagnose oligohydramnios in the United States,[1] many studies have found it to be an inaccurate method for assessing the actual amount of amniotic fluid, especially in the lower or higher ranges.[3,4,6,7,8,9] Rutherford et al.[10] point that poor intra- and interobserver reliability may account for some of the low positive predictive value. Serial measurements have shown mean differences of 1 cm of amniotic fluid volume when conducted by the same ultrasound operator, and 2 cm variance in measures of volume when conducted by multiple operators. Variation can exist because of subjectivity of the ultrasonographer, the amount of pressure applied to the abdomen, and fetal position or movement.[4] Additional variables that may alter AFI summation of the 4 uterine quadrants of amniotic fluid is the influence of the environmental temperature, altitude, maternal glucose control in diabetes, maternal hydration, and the status of the amniotic membranes.[6]

While error exists in simply determining the AFI, studies also show that the AFI is a poor predictor of perinatal outcome.[1,6,11] In women who have idiopathic oligohydramnios without other obstetric complications, there is no increase in adverse perinatal outcomes when compared to women who do not have oligohydramnios.[14] Although a low AFI does not reliably detect adverse pregnancy outcomes, it may lead to an increase in obstetric interventions without improving neonatal or maternal outcomes.[1,6,11,12,13]

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