Amniotic Fluid - 2 - Assessment of Amniotic Fluid Volume
With the widespread use of ultrasound, we have come to use ‘semi-objective’ measures of amniotic fluid volume (AFV). Although what is “abnormal” has gone through various permutations over time, most practitioners assess AFV by one of two methods. The first method is to simply measure the depth of the single largest vertical pocket of fluid (free of umbilical cord and body parts) with the ultrasound transducer oriented perpendicular to the uterus and not just perpendicular to the abdominal wall and with a minimum of pressure on the transducer itself. If the single largest pocket is 8 cm, the AFV is considered to be ‘increased’.
The second method that has gained perhaps the most popularity over time is the “amniotic fluid index (AFI).” This technique involves dividing the uterus into four ‘quadrants’ by imagining perpendicular lines running through the umbilicus of the mother, and then simply measuring the single deepest pocket of fluid in each quadrant and adding up the 4 results. At term, the mean AFI is 11.5 cm with 5th and 95th percentiles of 6.8 and 19.6 cm, respectively, and quite frankly, there is not much variation in those values from late midtrimester on. Oligohydramnios is then defined as an AFI 95th percentile.
Either approach can be performed quickly and though the AFI technique would intuitively seem to give you a better total picture of AFV, in reality, there probably is no real difference in the reliability of the methods. Indeed, in experienced hands, subjective assessment of amniotic fluid by simply looking is probably just as good (may the lord of evidence-based medicine please not strike me down)! If there are only very small pockets of free fluid, the baby appears crowded, or in the most extreme cases, there is absolutely no fluid (severe oligohydramnios or anhydramnios) and the baby has gross pressure deformations of the head or extremities, or overlapping of the ribs, the diagnosis of too little fluid is readily apparent. In a similar vein, if the baby appears to be freely floating or turning and has no restriction of extremity extension, there is probably too much fluid. With all that said and done, since not everyone has ‘experienced hands’ and there is some beauty to numbers, we use the 4-quadrant AFI as our routine approach to the assessment of AFV.
In the next post in this series we will discuss specific conditions and complications associated with decreased amniotic fluid….