### Description

Abnormally low (oligohydramnios) or high (polyhydramnios) amounts of amniotic fluid are associated with poor fetal outcomes. Identification of infants with low amniotic fluid index can aid in the decision for early intervention. The amount of amniotic fluid increases then declines over the course of pregnancy, so that the week of gestation needs to be known for accurate interpretation.

Measurement:

An ultrasound examination of the uterus is performed and the uterus is divided into four quadrants based on a line through a midline sagittal line and a transverse line midway through the uterus. The diameter of the pocket of amniotic fluid in each quadrant with the deepest vertical dimension is measured in millimeters, with the exception of pockets confluent with a pocket in another quadrant. The values for each quadrant is then summated.

amniotic fluid index =

= (diameter of left upper quadrant in mm) + (diameter of right upper quadrant in mm) + (diameter of left lower quadrant in mm) + (diameter of right lower quadrant in mm)

 week 5th percentile 50th percentile 95th percentile 16 79 121 185 17 83 127 194 18 87 133 202 19 90 137 207 20 93 141 212 21 95 143 214 22 97 145 216 23 98 146 218 24 98 147 219 25 97 147 221 26 97 147 223 27 95 146 226 28 94 146 228 29 92 145 231 30 90 145 234 31 88 144 238 32 86 144 242 33 83 143 245 34 81 142 248 35 79 140 249 36 77 138 249 37 75 135 244 38 73 132 239 39 72 127 226 40 71 123 214 41 70 116 194 42 69 110 175

from Moore and Cayle (1990)

When the data above is analyzed in JMP, the following equations are obtained:

50th percentile of amniotic fluid index =

= (-0.000744 * ((weeks gestation) ^ 4)) + (0.0868592 * ((weeks gestation) ^ 3)) - (3.860524 * ((weeks gestation) ^ 2)) + (76.795606 * (weeks gestation)) - 426.9041

5th percentile of amniotic fluid index =

= (0.0000265 * ((weeks gestation) ^ 4)) + (0.0058664 * ((weeks gestation) ^ 3)) - (0.756508 * ((weeks gestation) ^ 2)) + (24.64537 * (weeks gestation)) - 148.1449

95th percentile of amniotic fluid index =

= (-0.00258 * ((weeks gestation) ^ 4)) + (0.2766735 * ((weeks gestation) ^ 3)) - (10.92983 * ((weeks gestation) ^ 2)) + (190.93511 * (weeks gestation)) - 1036.273

The weekly change in amniotic fluid index is given by the following equation:

per cent change in amniotic fluid index =

= 12 - (0.27 * (weeks gestation)) - (0.005 * ((weeks gestation) ^ 2))

Assessment:

• Polyhydramnios can occur with fetal anomalies of the esophagus and gastrointestinal tract, with hemolytic disease of the newborn, with open neural tube defects, with chromosomal abnormalities or with maternal diabetes.

• Oligohydramnios can occur with decreased production of amniotic fluid (renal dysplasia or agenesis, urinary tract outlet obstruction) or loss (premature ruptured membranes).