Three-dimensional method for determination of amniotic fluid volume in intrauterine pockets

Background: Although current ultrasound techniques provide a linear (amniotic fluid index; AFI) or two-dimensional area index of amniotic fluid (AF), these indices have limited correlation with actual AF volume. We sought to quantify the three-dimensional volume of ultrasound-identified AF pockets, as assessed by the AFI and two-dimensional area methods. The BVI 2500 (Bladder Volume Instrument 2500; Diagnostic Ultrasound Corp., Redmond, WA) has been used to quantify the volume of residual urine in the bladder. Instrument and Method: The BVI 2500 (Diagnostic Ultrasound Corp.) ultrasound uses a rotating 2-MHz transducer, computer-defined fluid interface, and computer integration of 12 cross-sectional images to calculate three-dimensional fluid volume. After providing written informed consent, 14 term pregnant patients (36-42 weeks) were evaluated using the BVI 2500 and an Ultramark 8 sector scan (Advanced Technology Laboratory, Bothell, WA). The largest vertical fluid pocket in each quadrant of the abdomen was identified with the sector scan, and vertical and horizontal measurements for AFI and two-dimensional area were recorded. Simultaneous AF volume measurements of each pocket were performed three times with the bladder volume instrument, and maximum values were used. Three-dimensional volume, two-dimensional area, and AFI values were compared by correlation analysis, with P less than or equal to .05 considered statistically significant. Experience: Among all patients, the average (+/- standard deviation) AFI was 7.6 +/- 4.1 (range 1.5-16.4) cm, and the average two-dimensional area was 30.9 +/- 21.1 (range 4.3-81.3) cm(2). This corresponded to an average three-dimensional volume of 215 +/- 134 (range 23-497) cm(3). Three-dimensional volume correlated highly with both AFI (r = 0.9; P <.001) and two-dimensional area (r = 0.86; P <.001). One AFI centimeter was equivalent to a volume of 30 cm(3). Conclusions: There are highly significant linear correlations of three-dimensional amniotic fluid volumes with AFI and two-dimensional area. The four pockets used in AFI determination account for only 50% of total AF volume. Three-dimensional determinations may aid in clinical assessments of AF volume. (C) 1997 by The American College of Obstetricians and Gynecologists.