The amniotic fluid index in normal human pregnancy Thomas R. Moore, MD, and Jonathan E. Cayle, MD San Diego, California The four-quadrant sum of amniotic fluid pockets (amniotic fluid index) was studied prospectively in 791 normal pregnancies. Interobserver and intraobserver variation was 3.1% and 6.7%, respectively. Logarithmic transformations were used to establish the mean and 90% confidence intervals for the amniotic fluid index at each week of gestation. In term pregnancies, the boundaries of the amniotic fluid index were 115 mm (mean), 68 to 196 mm (5th to 95th percentiles). In postdates pregnancies >42 weeks, the values were 108 mm (mean), 67 to 174 mm (5th to 95th percentiles), p < 0.0001. However, the values for each week were statistically distinct, indicating the need to reference amniotic fluid index measurements to week-specific normative tables for accurate interpretation. This study provides normative data for the amniotic fluid index thoughout pregnancy. (AM J CasTET GVNECOL 1990;162:1168-73.)
Key words: Oligohydramnios, amniotic fluid, polyhydramnios Abnormalities of amniotic fluid volume are associated with poor pregnancy outcome. I -3 However, a convenient and reproducible method for estimation of amniotic fluid volume is lacking. Queenan et al.: Charles et al.,5 and Haswell and Morris 6 have measured amniotic fluid volume at different periods of gestation with indicator dilution techniques. However, these methods are oflimited clinical usefulness because of the necessity for amniocentesis to instill the indicator. Recently, ultrasonographic visualization of amniotic fluid has given rise to both subjective and semiquantitative methods of fluid estimation. Qualitative approaches permit the experienced sonographer to visually integrate multiple pockets of amniotic fluid into a subjective total that is compared with expected normal levels at each stage of gestation. These studies have demonstrated good correlation between extremes of amniotic fluid volume and fetal outcome/' 8 but the reliability and reproducibility of subjective scales are heavily dependent on operator experience. Semiquantitative approaches typically estimate amniotic fluid volume by measurement of the depth or width of the largest clear amniotic fluid pocket. I. 2. 9 These techniques also have demonstrated correlation between "abnormal" amniotic fluid volume and adverse fetal outcome, but the definitions of normal have been somewhat arbitrary. Moreover, the pocket-depth scales From the Department of Obstetrics and Gynecology, U.S. Naval Hospital, San Diego. The opinions or assertions expressed herein are those of the authors and are not to be construed as official or as reflecting the views of the United States Navy or the Naval Service at large. Received for publication August 23, 1989; revised January 25, 1990; accepted February 6, 1990. Reprint requests: Thomas R. Moore, MD, Department of Reproductive Medicine, University of California San Diego, H-813, 225 Dickinson St., San Diego, CA 92103. 611119913
1168
have been applied uniformly to a broad range of gestational ages, despite the fact that indicator-dilution studies have demonstrated that amniotic fluid volume varies significantly with pregnancy duration. 4 • 10 Finally, all previous studies have been conducted among a mixture of normal and abnormal pregnancies, rendering the task of defining normal amniotic fluid volume even more difficult. 11 In this study we investigated the use of an ultrasonographically derived index of volume obtained from measurements in four quadrants of the uterus. The aim of the study was to establish the normal range of the amniotic fluid index values across gestation in normal pregnancies, with determination of gestational age-specific boundaries of "normal" for each week. This article addresses the construction of a normal curve for amniotic fluid index in human pregnancy.
Material and methods Seven hundred ninety-one patients with normal pregnancies were evaluated prospectively. Normal patients were entered into the study according ot the criteria in Table I. Regardless of the indication for the sonogram in which amniotic fluid index was measured, if the pregnancy outcome and ultrasonographic results were normal, patients were included. Pregnancy outcomes were considered normal if birth weight was between the 10th and 90th percentiles according to California standards,12 no fetal anomalies were present, and the 5-minute Apgar score was above 6. The indications for ultrasonography are listed in Table II. Patients with twin gestation, ruptured membranes, fetal anomalies, confirmed fetal growth disorders, abnormal neonatal outcome, coexisting maternal diseases, or gestational age less than 16 weeks were excluded. The study was cross-sectional. Only a single examination from each pregnancy was included.
Normal values of amniotic fluid index
Volume 162 Number 5
1169
Table I. Amniotic fluid index study: inclusion criteria Amniotic fluid index measurement Normal pregnancy outcome
Adequate obstetric dates
Between 16 and 44 wk No ultrasonographic abnormalities Term delivery Birth weight 10th to 90th percentile Normal newborn course Normal and certain last menstrual period and
Crown-rump length at 7-11 wk* or
Ultrasonogram 16-24 wkt *Crown-rump length must be within I week of last menstrual period dates. tAverage of biometric measurements must be within 1.5 weeks of last menstrual period dates. The technique of obtaining the amniotic fluid index is outlined in Table III. Ultrasonographic examinations were perforemd with an Ultramark IV model (A DR, Bothell, Wash.) with linear transducers (3.5 and 5 MHz). The uterus was divided into four quadrants along the sagittal midline and midway up the fundus. The transducer was then maintained in a vertical and sagittal alignment, and the pocket of amniotic fluid with the deepest vertical dimension was identified and measured. This procedure was repeated in each quadrant and the values were summed. Pockets confluent with pockets in adjacent quadrants were avoided. Amniotic fluid estimations were reported to the clinician ordering the study, but patient management was altered only if no pockets larger than I cm in depth were identified. The reproducibility of the measurements between different observers (interobserver variation) and between measurements taken by a single observer (intraobserver variation) was tested in 20 and 50 cases, respectively. Five sonographers participated in the interobserver study; two sonographers performed the intraobserver study. The "error" in multiple measurements was calculated by comparing each single measurement with the average of three measurements obtained by either a single or three separate sonographers. Data were analyzed by means of a microcomputerbased statistical package. Amniotic fluid index observations were stratified into gestational week segments. Because the amniotic fluid index values in each gestational period were not normally distributed, the data were transformed into logarithmic (base 10) values for further analysis. The 90%, 95%, and 98% confidence intervals about the means were determined for each period of gestation. The mean amniotic fluid index values in patient subgroups (preterm, term, postterm) were compared with a two-tailed t test. Results obtained from the log 10 transforms were reconverted into their antilogs for graphic display. Curves for the confidence intervals were plotted from fourth-order polynomial equations derived from regressing the transformed percentile data against gestational age.
Table II. Indications for ultrasonographic examination Indication
n
%
Com firm dates Postdates fluid assessment Suspected genetic disorder Suspected preterm labor Placental localization Suspected growth delay
248 194 124
31 25 16
106 88 31
II
13 4
Table III. Method of determination of amniotic fluid index l5 Patient positioned supine Uterus viewed as four equal quadrants Ultrasound transducer perpendicular to plane of the floor and aligned longitudinally with the patient'S spine Vertical depth of the largest clear amniotic fluid pocket is measured in millimeters Amniotic fluid index = sum of four quadrant pocket depths
Table IV. Interobserver and intraobserver variation in amniotic fluid index
Intraobserver Interobserver
Error (mm) (Mean + SEM)
Percent en-or (Mean + SEM)
5.00 ± 1.2 9.66 ± 0.7
3.07 ± 0.7 6.66 ± 0.6
Results The interobserver and intraobserver variability data are presented in Table IV. The absolute errors (in millimeters) for interobserver and intraobserver assessments represent a variation of approximately 7% and 3% from the average amniotic fluid index, respectively. Fig. I demonstrates the relationship between the absolute errors (millimeters) and percent errors generated by three amniotic fluid index measurements by a
70
Moore and Cayle
May 1990 Am J Obstet Gynecol
_
Ernr - 11.7
+/- 5.2 .....
_
" Err... - 8.7
+/-
4 "
2D
·0
"
00
-------~~--~------aa a
0"'. 00
D
•
'00'110
____
Average AFI
_
(""">
•
D
100
0
0
150
2DO
110
Average AFI
(""">
»0
3ISO
400
Fig. 1. Intraobserver variation in amniotic fluid index. A, Closed squares represent the absolute error in millimeters for an individual observation compared with the average of three made by a single observer. B, Open squares represent the percentage difference between individual and three averaged measures of amniotic fluid index. Dashed lines indicate the mean error. 350
300
- ~ ", :~::~41 weeks) are also shown. The amniotic fluid index values for the three subgroups are statistically distinct (p < 0.(001). When the term and postdates groups are compared, the lower boundaries (oligohydramnios) are similar but the upper limits of amniotic fluid index (polyhydramnios) are much higher (196 mm = 95th percentile for term versus 174 mm postdates). The amniotic fluid index values
for all percentiles are higher in preterm gestations than those observed in term or postterm pregnancies (p < 0.00(1). Because the amniotic fluid index measurements varied with the gestational age at observation, the data were stratified by week of pregnancy. The means, 90%, and 95% confidence intervals of the amniotic fluid index observations are listed in Table VI. The variation in amniotic fluid index over gestation was highly statistically significant (analysis of variance, p < 0.000 I). Fig. 2 illustrates the changes in amniotic fluid index from 16 to 43 weeks, together with the 90% and 98% confidence intervals around the mean. The mean amniotic fluid index curve rises slowly from 16 to 27 weeks, plateaus until 33 weeks, then declines steadily into the postdates period. The contour of the mean amniotic fluid index curve was subjected to further analysis by determining the rate of change in amniotic fluid index between succes-
1172 Moore and Cayle
May 1990 Am J Obstet Gynecol
Table VI. Amniotic fluid index values in normal pregnancy Amniotic fluid index percentile values Week
2.5th
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
73 77 80 83 86 88 89 90 90 89 89 85 86 84 82 79 77 74 72 70 68 66 65 64 63 63 63
I
5th
I
50th
79 83 87 90 93 95 97 98 98 97 97 95 94 92 90 88 86 83 81 79 77
75 73 72 71 70 69
sive weeks of gestation. Fig. 3 plots the percentage change in the mean amniotic fluid index compared with the amniotic fluid index of the previous week. The curve has a continuously negative slope, crossing 0 at 27 weeks. Thus the amniotic fluid index increases before 27 weeks but at a steadily decreasing rate. During this time amniotic fluid index increases by approximately 25% from 16 to 27 weeks. After 27 weeks the amniotic fluid index declines at a progressively increasing rate. By the end of the third trimester the amniotic fluid index has dropped by approximately 29% from the 27-week peak. In the postdates period the amniotic fluid index decreases by approximately 12% per week. Comment
This prospective evaluation of the amniotic fluid index in normal pregnancy from 16 to 43 weeks demonstrated that amniotic fluid index measurements are highly reproducible, with an interobserver error of 7% and an intraobserver variation of 3%. Our findings in absolute interobserver and intraobserver variation (9.7 mm and 5 mm) are similar to the report by Rutherford et al.,13 which demonstrated errors of 1.0 and 2.0 cm, respectively. However, our data indicate that the variation in amniotic fluid index measurements is not constant with regard to the amount of amniotic fluid presen~: Amniotic fluid index measurements obtained in the setting of oligohydramnios require cautious interpretation. We recommend averaging three amniotic fluid index measurements when the index is below 100 mm.
121 127 133 137 141 143 145 146 147 147 147 146 146 145 145 144 144 143 142 140 138 135 132 127 123 116 110
I
95th
185 194 202 207 212 214 216 218 219 221 223 226 228 231 234 238 242 245 248 249 249 244 239 226 214 194 175
I
97.5th
n
201 211 220 225 230 233 235 237 238 240 242 245 249 254 258 263 269 274 278 279 279 275 269 255 240 216 192
32 26 17 14 25 14 14 14
23 12 11 17 25 12 17 26 25 30 31 27 39 36 27 12 64 162 30
The amniotic fluid index observations at each gestational period were not normally distributed, with significant splay into the upper ranges. Logarithmic transforms were used to calculate the gestation-specific averages and percentile ranks to ensure accuracy and predictability. The values thus obtained from the amniotic fluid index were statistically distinct. This suggests that amniotic fluid index measurements should be referenced to gestational age standards. Table VI and Fig. 2 permit easy reference to these standards. Phelan et al. I' also assayed amniotic fluid index values across a range of gestational ages in a group of 197 selected patients. Their work also demonstrated a peak in amniotic fluid index at 27 to 29 weeks, with a relative plateau until term. However, because logarithmic transforms were not applied to correct for skew, their calculations of "average amniotic fluid index" do not compare with our findings. However, when the algebraic mean of the amniotic fluid index values in the entire data set is calculated, our value of 139 mm correlates fairly well with their value of 10.5 to 16.5 cm. H • 15 Other investigators have proposed ultrasonographic criteria for normal amniotic fluid volume. Manning et al. I6 incorporated a minimum pocket depth of 1 cm into the biophysical profile. Subsequently this criterion has been criticized as too restrictive and lacking in sensitivity.I7 Chamberlain et al. I. 2 assigned a scale to the vertical depth of the largest visible amniotic fluid pocket: >8 cm constitutes polyhydramnios, 1 cm represents mild oligohydramnios, and < 1 cm
Normal values of amniotic fluid index
Volume 162 Number 5
indicates severe oligohydramnios. These limits equated to the 97th, second, and first percentiles, respectively, in their study of over 7000 selected pregnancies. Although extremes of vertical pocket depth correlated well with outcome in those studies, restricting the definition of "abnormal" to 1% to 3% of the total may reduce sensitivity of the test. Moreover, the lack of adjustment for gestational age differences in amniotic fluid volume may impair the effectiveness of this scale. 11. 17 Rutherford et al. 15 have suggested a "5 cm rule" as the lower limits of acceptability for the amniotic fluid index in term gestation. However, the rationale for this limit has not been rigorously established. Although their studies demonstrate significant morbidity in pregnancies with amniotic fluid index values
Key words: Oligohydramnios, amniotic fluid, polyhydramnios Abnormalities of amniotic fluid volume are associated with poor pregnancy outcome. I -3 However, a convenient and reproducible method for estimation of amniotic fluid volume is lacking. Queenan et al.: Charles et al.,5 and Haswell and Morris 6 have measured amniotic fluid volume at different periods of gestation with indicator dilution techniques. However, these methods are oflimited clinical usefulness because of the necessity for amniocentesis to instill the indicator. Recently, ultrasonographic visualization of amniotic fluid has given rise to both subjective and semiquantitative methods of fluid estimation. Qualitative approaches permit the experienced sonographer to visually integrate multiple pockets of amniotic fluid into a subjective total that is compared with expected normal levels at each stage of gestation. These studies have demonstrated good correlation between extremes of amniotic fluid volume and fetal outcome/' 8 but the reliability and reproducibility of subjective scales are heavily dependent on operator experience. Semiquantitative approaches typically estimate amniotic fluid volume by measurement of the depth or width of the largest clear amniotic fluid pocket. I. 2. 9 These techniques also have demonstrated correlation between "abnormal" amniotic fluid volume and adverse fetal outcome, but the definitions of normal have been somewhat arbitrary. Moreover, the pocket-depth scales From the Department of Obstetrics and Gynecology, U.S. Naval Hospital, San Diego. The opinions or assertions expressed herein are those of the authors and are not to be construed as official or as reflecting the views of the United States Navy or the Naval Service at large. Received for publication August 23, 1989; revised January 25, 1990; accepted February 6, 1990. Reprint requests: Thomas R. Moore, MD, Department of Reproductive Medicine, University of California San Diego, H-813, 225 Dickinson St., San Diego, CA 92103. 611119913
1168
have been applied uniformly to a broad range of gestational ages, despite the fact that indicator-dilution studies have demonstrated that amniotic fluid volume varies significantly with pregnancy duration. 4 • 10 Finally, all previous studies have been conducted among a mixture of normal and abnormal pregnancies, rendering the task of defining normal amniotic fluid volume even more difficult. 11 In this study we investigated the use of an ultrasonographically derived index of volume obtained from measurements in four quadrants of the uterus. The aim of the study was to establish the normal range of the amniotic fluid index values across gestation in normal pregnancies, with determination of gestational age-specific boundaries of "normal" for each week. This article addresses the construction of a normal curve for amniotic fluid index in human pregnancy.
Material and methods Seven hundred ninety-one patients with normal pregnancies were evaluated prospectively. Normal patients were entered into the study according ot the criteria in Table I. Regardless of the indication for the sonogram in which amniotic fluid index was measured, if the pregnancy outcome and ultrasonographic results were normal, patients were included. Pregnancy outcomes were considered normal if birth weight was between the 10th and 90th percentiles according to California standards,12 no fetal anomalies were present, and the 5-minute Apgar score was above 6. The indications for ultrasonography are listed in Table II. Patients with twin gestation, ruptured membranes, fetal anomalies, confirmed fetal growth disorders, abnormal neonatal outcome, coexisting maternal diseases, or gestational age less than 16 weeks were excluded. The study was cross-sectional. Only a single examination from each pregnancy was included.
Normal values of amniotic fluid index
Volume 162 Number 5
1169
Table I. Amniotic fluid index study: inclusion criteria Amniotic fluid index measurement Normal pregnancy outcome
Adequate obstetric dates
Between 16 and 44 wk No ultrasonographic abnormalities Term delivery Birth weight 10th to 90th percentile Normal newborn course Normal and certain last menstrual period and
Crown-rump length at 7-11 wk* or
Ultrasonogram 16-24 wkt *Crown-rump length must be within I week of last menstrual period dates. tAverage of biometric measurements must be within 1.5 weeks of last menstrual period dates. The technique of obtaining the amniotic fluid index is outlined in Table III. Ultrasonographic examinations were perforemd with an Ultramark IV model (A DR, Bothell, Wash.) with linear transducers (3.5 and 5 MHz). The uterus was divided into four quadrants along the sagittal midline and midway up the fundus. The transducer was then maintained in a vertical and sagittal alignment, and the pocket of amniotic fluid with the deepest vertical dimension was identified and measured. This procedure was repeated in each quadrant and the values were summed. Pockets confluent with pockets in adjacent quadrants were avoided. Amniotic fluid estimations were reported to the clinician ordering the study, but patient management was altered only if no pockets larger than I cm in depth were identified. The reproducibility of the measurements between different observers (interobserver variation) and between measurements taken by a single observer (intraobserver variation) was tested in 20 and 50 cases, respectively. Five sonographers participated in the interobserver study; two sonographers performed the intraobserver study. The "error" in multiple measurements was calculated by comparing each single measurement with the average of three measurements obtained by either a single or three separate sonographers. Data were analyzed by means of a microcomputerbased statistical package. Amniotic fluid index observations were stratified into gestational week segments. Because the amniotic fluid index values in each gestational period were not normally distributed, the data were transformed into logarithmic (base 10) values for further analysis. The 90%, 95%, and 98% confidence intervals about the means were determined for each period of gestation. The mean amniotic fluid index values in patient subgroups (preterm, term, postterm) were compared with a two-tailed t test. Results obtained from the log 10 transforms were reconverted into their antilogs for graphic display. Curves for the confidence intervals were plotted from fourth-order polynomial equations derived from regressing the transformed percentile data against gestational age.
Table II. Indications for ultrasonographic examination Indication
n
%
Com firm dates Postdates fluid assessment Suspected genetic disorder Suspected preterm labor Placental localization Suspected growth delay
248 194 124
31 25 16
106 88 31
II
13 4
Table III. Method of determination of amniotic fluid index l5 Patient positioned supine Uterus viewed as four equal quadrants Ultrasound transducer perpendicular to plane of the floor and aligned longitudinally with the patient'S spine Vertical depth of the largest clear amniotic fluid pocket is measured in millimeters Amniotic fluid index = sum of four quadrant pocket depths
Table IV. Interobserver and intraobserver variation in amniotic fluid index
Intraobserver Interobserver
Error (mm) (Mean + SEM)
Percent en-or (Mean + SEM)
5.00 ± 1.2 9.66 ± 0.7
3.07 ± 0.7 6.66 ± 0.6
Results The interobserver and intraobserver variability data are presented in Table IV. The absolute errors (in millimeters) for interobserver and intraobserver assessments represent a variation of approximately 7% and 3% from the average amniotic fluid index, respectively. Fig. I demonstrates the relationship between the absolute errors (millimeters) and percent errors generated by three amniotic fluid index measurements by a
70
Moore and Cayle
May 1990 Am J Obstet Gynecol
_
Ernr - 11.7
+/- 5.2 .....
_
" Err... - 8.7
+/-
4 "
2D
·0
"
00
-------~~--~------aa a
0"'. 00
D
•
'00'110
____
Average AFI
_
(""">
•
D
100
0
0
150
2DO
110
Average AFI
(""">
»0
3ISO
400
Fig. 1. Intraobserver variation in amniotic fluid index. A, Closed squares represent the absolute error in millimeters for an individual observation compared with the average of three made by a single observer. B, Open squares represent the percentage difference between individual and three averaged measures of amniotic fluid index. Dashed lines indicate the mean error. 350
300
- ~ ", :~::~41 weeks) are also shown. The amniotic fluid index values for the three subgroups are statistically distinct (p < 0.(001). When the term and postdates groups are compared, the lower boundaries (oligohydramnios) are similar but the upper limits of amniotic fluid index (polyhydramnios) are much higher (196 mm = 95th percentile for term versus 174 mm postdates). The amniotic fluid index values
for all percentiles are higher in preterm gestations than those observed in term or postterm pregnancies (p < 0.00(1). Because the amniotic fluid index measurements varied with the gestational age at observation, the data were stratified by week of pregnancy. The means, 90%, and 95% confidence intervals of the amniotic fluid index observations are listed in Table VI. The variation in amniotic fluid index over gestation was highly statistically significant (analysis of variance, p < 0.000 I). Fig. 2 illustrates the changes in amniotic fluid index from 16 to 43 weeks, together with the 90% and 98% confidence intervals around the mean. The mean amniotic fluid index curve rises slowly from 16 to 27 weeks, plateaus until 33 weeks, then declines steadily into the postdates period. The contour of the mean amniotic fluid index curve was subjected to further analysis by determining the rate of change in amniotic fluid index between succes-
1172 Moore and Cayle
May 1990 Am J Obstet Gynecol
Table VI. Amniotic fluid index values in normal pregnancy Amniotic fluid index percentile values Week
2.5th
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
73 77 80 83 86 88 89 90 90 89 89 85 86 84 82 79 77 74 72 70 68 66 65 64 63 63 63
I
5th
I
50th
79 83 87 90 93 95 97 98 98 97 97 95 94 92 90 88 86 83 81 79 77
75 73 72 71 70 69
sive weeks of gestation. Fig. 3 plots the percentage change in the mean amniotic fluid index compared with the amniotic fluid index of the previous week. The curve has a continuously negative slope, crossing 0 at 27 weeks. Thus the amniotic fluid index increases before 27 weeks but at a steadily decreasing rate. During this time amniotic fluid index increases by approximately 25% from 16 to 27 weeks. After 27 weeks the amniotic fluid index declines at a progressively increasing rate. By the end of the third trimester the amniotic fluid index has dropped by approximately 29% from the 27-week peak. In the postdates period the amniotic fluid index decreases by approximately 12% per week. Comment
This prospective evaluation of the amniotic fluid index in normal pregnancy from 16 to 43 weeks demonstrated that amniotic fluid index measurements are highly reproducible, with an interobserver error of 7% and an intraobserver variation of 3%. Our findings in absolute interobserver and intraobserver variation (9.7 mm and 5 mm) are similar to the report by Rutherford et al.,13 which demonstrated errors of 1.0 and 2.0 cm, respectively. However, our data indicate that the variation in amniotic fluid index measurements is not constant with regard to the amount of amniotic fluid presen~: Amniotic fluid index measurements obtained in the setting of oligohydramnios require cautious interpretation. We recommend averaging three amniotic fluid index measurements when the index is below 100 mm.
121 127 133 137 141 143 145 146 147 147 147 146 146 145 145 144 144 143 142 140 138 135 132 127 123 116 110
I
95th
185 194 202 207 212 214 216 218 219 221 223 226 228 231 234 238 242 245 248 249 249 244 239 226 214 194 175
I
97.5th
n
201 211 220 225 230 233 235 237 238 240 242 245 249 254 258 263 269 274 278 279 279 275 269 255 240 216 192
32 26 17 14 25 14 14 14
23 12 11 17 25 12 17 26 25 30 31 27 39 36 27 12 64 162 30
The amniotic fluid index observations at each gestational period were not normally distributed, with significant splay into the upper ranges. Logarithmic transforms were used to calculate the gestation-specific averages and percentile ranks to ensure accuracy and predictability. The values thus obtained from the amniotic fluid index were statistically distinct. This suggests that amniotic fluid index measurements should be referenced to gestational age standards. Table VI and Fig. 2 permit easy reference to these standards. Phelan et al. I' also assayed amniotic fluid index values across a range of gestational ages in a group of 197 selected patients. Their work also demonstrated a peak in amniotic fluid index at 27 to 29 weeks, with a relative plateau until term. However, because logarithmic transforms were not applied to correct for skew, their calculations of "average amniotic fluid index" do not compare with our findings. However, when the algebraic mean of the amniotic fluid index values in the entire data set is calculated, our value of 139 mm correlates fairly well with their value of 10.5 to 16.5 cm. H • 15 Other investigators have proposed ultrasonographic criteria for normal amniotic fluid volume. Manning et al. I6 incorporated a minimum pocket depth of 1 cm into the biophysical profile. Subsequently this criterion has been criticized as too restrictive and lacking in sensitivity.I7 Chamberlain et al. I. 2 assigned a scale to the vertical depth of the largest visible amniotic fluid pocket: >8 cm constitutes polyhydramnios, 1 cm represents mild oligohydramnios, and < 1 cm
Normal values of amniotic fluid index
Volume 162 Number 5
indicates severe oligohydramnios. These limits equated to the 97th, second, and first percentiles, respectively, in their study of over 7000 selected pregnancies. Although extremes of vertical pocket depth correlated well with outcome in those studies, restricting the definition of "abnormal" to 1% to 3% of the total may reduce sensitivity of the test. Moreover, the lack of adjustment for gestational age differences in amniotic fluid volume may impair the effectiveness of this scale. 11. 17 Rutherford et al. 15 have suggested a "5 cm rule" as the lower limits of acceptability for the amniotic fluid index in term gestation. However, the rationale for this limit has not been rigorously established. Although their studies demonstrate significant morbidity in pregnancies with amniotic fluid index values
