Amniotic fluid provides a supportive and protective environment for fetal development during pregnancy. Amniotic 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. Owing to the frequent use of bedside ultrasound, much is known about the regulation of and normative values for amniotic fluid volume and the mechanisms by which this fluid is regulated.
The following Summary Statements and Recommendation are mentioned in Technical Update (“Amniotic Fluid: Technical Update on Physiology and Measurement” published January 2017) which has been prepared by the Diagnostic Imaging committee, reviewed by the Society of Obstetricians and Gynaecologists of Canada (SOGC) Guideline Management and Oversight Committee and approved by the Board of the SOGC. To access the full text of this article, please contact JOGC (Journal of Obstetrics and Gynaecology Canada) or SOGC (Society of Obstetricians and Gynaecologists of Canada).
- 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)
- Accurate quantification of amniotic fluid volumes using current ultrasound technology remains challenging. (II-2)
- 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)
- 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)
- 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)
- There is insufficient literature to determine which method of amniotic fluid assessment is more reproducible than the other. (II-1)
- 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)
- 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)