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).
Specialty: Obstetrics & Gynecology
ICD-10: ,
