Amniotic Fluid: Technical Update on Physiology and Measurement

Objective1.To provide an update on the use of ultrasound to evaluate amniotic fluid volume.2.To provide an update on amniotic fluid physiology.3.To promote evidence based assessment techniques and definitions for amniotic fluid volume.Outcomes1.Reduced interventions as a result of the diagnosis of oligohydramnios without increasing adverse outcomes.2.By understanding the limitations of amniotic fluid assessment, promote more efficient use of ultrasound assessment.EvidenceA MEDLINE and KFINDER search was used to identify relevant articles, with review of bibliography identified article including Cochrane reviews and recent review articles.ValuesThe evidence collected was reviewed by the Diagnostic Imaging Committee of the Society of Obstetricians and Gynecologists of Canada. The recommendations were made according to the guidelines developed by The Canadian Task Force on Preventative Health Care (Table 1).Benefits, Harms and CostsAmniotic fluid assessment by ultrasound has become an integral part of fetal assessment in modern obstetrics. Abnormalities of fluid volume result in obstetrical intervention and further investigations. In Canada, there are no standard definitions of fluid volume estimation, nor a standard approach to assessing fluid. Multiple randomized trials have suggested that using a Single Pocket Estimation technique (rather than the multi pocket assessment approach known as the amniotic fluid index), will result in fewer obstetrical interventions without any increase in adverse outcomes. Recent literature suggests that there are detectable, modest changes in amniotic fluid that can occur within an hour or two of normal physiological maneuvers. This may account for the variability and inconsistent results from repeated assessments within a short period of time which can lead to confusion and generate further testing. This article hopes to describe the limitations of amniotic fluid assessment, promote a standard method of amniotic fluid assessment, and propose a common set of definitions to be used to describe amniotic fluid volume.Summary Statements1.Changes in amniotic fluid volume are usually gradual, however modest shifts can occur within hours due to hydration, maternal positioning, and/or activity. Water channels such as aquaporins likely facilitate these rapid changes. (I and II-2)2.Accurate quantification of amniotic fluid volumes using current ultrasound technology remains challenging. (II-2)3.Various techniques for single pocket estimation of amniotic fluid are described in the literature. Most of these descriptions are open to interpretation, and no particular method has been shown to be superior to the other in direct comparison. (III)4.Using the single pocket estimation method of amniotic fluid assessment in the third trimester to diagnose oligohydramnios, rather than the amniotic fluid index method, will result in fewer interventions without increasing adverse perinatal outcomes. (I)5.Polyhydramnios as defined using amniotic fluid index is associated with a variety of adverse outcomes. More studies are needed looking at outcomes of polyhydramnios using the single pocket estimation definitions of polyhydramnios. (II-2)6.There is insufficient literature to determine which method of amniotic fluid assessment is more reproducible than the other. (II-1)7.It is proposed that the Chamberlain classification of amniotic fluid be used to define oligohydramnios (single deepest pocket [SDP] smaller than 2 cm in depth × 1 cm wide) and polyhydramnios (SDP greater than 8 cm in depth × 1 cm wide) for the initial assessment of amniotic fluid during routine obstetrical scanning. (I)Recommendation1.It is recommended that the initial, general evaluation of amniotic fluid volume during routine obstetrical ultrasound be a single pocket estimation. The Chamberlain method of amniotic fluid assessment is the preferred method for estimation. (I-A)