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Results: Clinical Obstetrics and Gynecology
(C) Lippincott-Raven Publishers
Volume 40(2), June 1997, pp 303-313
Clinical Assessment of Amniotic Fluid
[Articles]
MOORE, THOMAS R. MD
Department of Reproductive Medicine, University of California San Diego
Correspondence: Thomas R. Moore, MD, Mail Code 8433, 200 West Arbor Drive,
San Diego, CA 92118.
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Outline
Abstract
Clinical Value of Amniotic Fluid Volume Assessment
PREDICTION OF POOR PERINATAL OUTCOME
DETECTION OF FETAL ANOMALIES
IDENTIFICATION OF INTRAUTERINE GROWTH RESTRICTION AND PLACENTAL
INSUFFICIENCY
Factors Influencing Amniotic Fluid Volume
AMNIOTIC FLUID PRODUCTION
AMNIOTIC FLUID REMOVAL
Gestational Age Influences on Amniotic Fluid Volume
Techniques of Estimating Amniotic Fluid Volume
INTERSERVER AND INTRAOBSERVER RELIABILITY
TECHNICAL ASPECTS OF PERFORMING THE AMNIOTIC FLUID INDEX
Indications for and Frequency of Amniotic Fluid Volume Assessment
Summary
References
Graphics
Fig. 1
Table 1
Table 2
Fig. 2
Table 3
Fig. 3
Abstract
Appreciation of the importance of amniotic fluid volume as an indicator of
fetal status is a relatively recent development.1 Before 1975, discussions
of amniotic fluid volume in the obstetric literature were limited to
observations of the quantity of fluid released after rupture of membranes.
The occurrence of thick meconium and fetal distress in post dates
pregnancy, for example, was attributed vaguely to "placental
insufficiency." More recently, progressive improvements in ultrasonographic
imaging have taken the technology of fetal and amniotic fluid assessment
from the stage of subjective impression to the present state in which
relatively sophisticated judgments of fetal condition can be based on
reproducible measurements.
In present practice, semiquantitative amniotic fluid volume assessment
during routine ultrasound (US) examination and antepartum testing has
become the standard of care. However, the complicated relationships imposed
by the placenta and complexly folded fetus within an irregularly ovoid
uterus have impeded the development of a precise method of calculating
amniotic fluid volume ultrasonographically. And although both subjective
and semiquantitative methods of estimating amniotic volume are in use, the
best technique remains controversial. In this article, the author reviews
the relative precision of the various volume estimation techniques and
clinical situations in which amniotic fluid volume assessment is helpful.
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Clinical Value of Amniotic Fluid Volume Assessment
PREDICTION OF POOR PERINATAL OUTCOME
Recognizing abnormal amniotic fluid volume before delivery may alert the
clinician to situations of potentially high perinatal risk. Chamberlain et
al.2 observed a perinatal mortality rate of 4.12/1,000 in pregnancies with
polyhydramnios compared with a rate of 1.97/1,000 when the amniotic fluid
was
normal. The perinatal mortality rate was increased 13-fold more than normal
when amniotic fluid volume was sonographically marginal, and increased
47-fold
(187.5/1,000) if severe oligohydramnios was present.
Pregnancies complicated by extremes of amniotic fluid volume also
experience increased maternal and neonatal morbidity. During labor,
polyhydramnios is associated with abnormal fetal lie, operative delivery,
and abruptio placentae.3 Preterm delivery occurred in 11.1% in patients
with polyhydramnios studied by Varma et al.4 compared with 6.7% in controls
with normal fluid. Fetal distress, low Apgar scores, macrosomia, and
intensive care nursery admission were significantly more frequent in the
polyhydramnios group.
With oligohydramnios, meconium, fetal heart rate abnormalities, and
depressed Apgar scores are more frequent: neonatal (31.2%) and fetal (25.0%)
acidosis
rates were doubled compared with controls;5 fetal distress requiring
operative intervention was tripled (64%) with oligohydramnios compared with
21% of
normals (P = .005).6 Crowley et al.7 reported meconium staining in 29% and
an emergency cesarean section rate of 11% with oligohydramnios in post-date
patients but only 2% in normals. Maternal complications of oligohydramnios
include increased incidence of hypertension (22.1%), second trimester
bleeding (4.1%), and abruptio placentae (4.2%).8
DETECTION OF FETAL ANOMALIES
Recognition of abnormal amniotic fluid volume may provide clues to
congenital anomalies, which might otherwise be overlooked. The finding of
polyhydramnios may lead to detection of fetal gastrointestinal obstruction
(esophageal
atresia, or thoracic masses compressing the esophagus such as diaphragmatic
hernia).4 Cardiac, intracranial, spinal, and ventral wall anomalies have
also been reported with excessive amniotic fluid.9 Oligohydramnios is
associated with increased incidence of fetal urinary tract abnormalities,
including
uretero-pelvic junction, uretero-vesical and posterior urethral obstruction,
polycystic kidneys, and renal agenesis.10 Long-standing oligohydramnios
restricts fetal movements, predisposing to compression orthopedic
abnormalities and interfering with normal fetal lung development, resulting
in lethal or sublethal pulmonary hypoplasia.11
IDENTIFICATION OF INTRAUTERINE GROWTH RESTRICTION AND PLACENTAL
INSUFFICIENCY
Oligohydramnios may be a sign of poor placental function. Oligohydramnios
is frequently associated with intrauterine growth retardation, intrapartum
asphyxia, and fetal demise because fetal urinary flow is determined in part
by the state of fetal hydration, which is in turn determined by placental
function. When serial amniotic fluid volume studies were performed on
hypertensive women in late pregnancy, intrauterine growth retardation
frequency was inversely proportional to amniotic fluid volume.12
Acute decreases in amniotic fluid volume may signal worsening of maternal
hypertension and declining placental function. When Cruz et al.13 compared
Doppler studies in patients with oligohydramnios and normals, those with
decreased fluid had significantly higher flow resistances in both the
maternal uterine and fetal umbilical arteries, suggesting that decreased
amniotic fluid is evidence of inadequate placental perfusion. In postdate
pregnancies, Tongsong et al.14 noted that amniotic fluid volume was
significantly more accurate in predicting intrapartum fetal distress than
the nonstress test (NST) with sensitivity, specificity, and positive
predictive values of 72.73%, 90.87%, and 26.67%, respectively. Hashimoto et
al.15 also reported a strong association between sonographically detected
oligohydramnios and the fetal postmaturity syndrome.
Factors Influencing Amniotic Fluid Volume
AMNIOTIC FLUID PRODUCTION
Fetal urination is the major source of amniotic fluid after fetal kidney
function begins at 10-12 weeks.16 Animal studies indicate that fetal
urinary flow rate at term is copious (200 ml/kg daily).17 Human studies
using using serial US measurements of fetal bladder dimensions have
suggested an even higher rate of 1,200 ml daily.18 Considering that
amniotic fluid volume is approximately 800 ml at term, it is likely that
the normal fetus turns over the entire volume of amniotic fluid in less
than 24 hours. Changes in fetal urinary flow rate can therefore be expected
to have a major impact on amniotic fluid volume. However,
at present the clinical situations in which amniotic fluid volume is
affected by changes in fetal renal function are as yet unclear.
Nevertheless, clinical evaluation of oligohydramnios or polyhydramnios
should consider fetal renal anatomy and function as a key factor.
Fetal lung fluid is a minor contributor to amniotic fluid volume. In the
near term sheep, lung fluid amounts to approximately 400 ml daily. However,
because much of the fluid leaving the trachea is swallowed immediately,
only approximately half reaches the amniotic cavity.19 In human fetuses,
lung fluid contribution to amniotic fluid is probably even less significant
because very little amniotic fluid is present with renal agenesis.
AMNIOTIC FLUID REMOVAL
Fetal swallowing is the major path by which fluid is removed from amniotic
cavity. Pritchard's 20 measurements of human fetal swallowing using
radiolabeled erythrocytes injected into the amniotic cavity indicate a
swallowing rate of approximately 500 ml daily. This estimate, similar to the
rates per kilogram obtained in laboratory animals, is far less than the
approximately 1,000 ml entering the amniotic space daily via the kidneys,
which suggests that another
pathway of fluid removal must be operative in the regulation of amniotic
fluid volume.
Other pathways. Other potential mechanisms that may help balance the excess
fluid entering the amniotic space include transmembranous movement (across
the membranes into the maternal circulation) and intramembranous movement
(into the fetal circulation via the blood vessels on the fetal surface of
the placenta). Although neither of these pathways has been completely
explored in human pregnancies, both have been shown in animal models.21 It
is likely that transmembranous fluid resorption plays a major role in
amniotic fluid volume regulation in all mammalian species.
Gestational Age Influences on Amniotic Fluid Volume
The most accurate picture of amniotic fluid volume changes during human
gestation is provided in a compilation of 705 published observations of
amniotic volumes in normal pregnancies ranging from 8-43 weeks gestation by
Brace and Wolf.22 Their findings are shown in Figure 1.
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FIG. 1. Amniotic fluid volume as a function of gestational age. The
shaded area covers 95% confidence interval. Reproduced with permission from
Brace RA, Wolf EJ. Normal amniotic fluid volume changes throughout
pregnancy. Am J Obstet Gynecol. 1989; 161:382-388.
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Several points about amniotic fluid trends during pregnancy are illustrated
by Figure 1.
1. Amniotic fluid volume (AFV) rises progressively during gestation until
approximately 32 weeks.
2. From 32 to term, the mean AFV is relatively constant, in the range of
700-800 ml.
3. After 40 weeks, there is a progressive decline in amniotic fluid volume
at a rate of 8%/wk, with amniotic fluid volume averaging only 400 ml at 42
weeks.
4. The variation in "normal" fluid volume below the mean value is smaller
than the upper variation in the third trimester. "Oligohydramnios" (defined
as the 5th percentile) is approximately 300 ml. However, variation in the
upper range is almost threefold greater, so that "hydramnios" (>95th
percentile) varies from 1700-1900 ml.
Techniques of Estimating Amniotic Fluid Volume
The optimal technique for sonographic estimation of amniotic fluid volume
should reproducibly estimate amniotic fluid volume and should correlate well
with abnormal fetal and maternal physiologic states. It should also be
simple enough to be learned and used readily clinically.
Subjective Assessment. This method, in which the relative amount of
echo-free fluid areas are subjectively compared with the space occupied by
the fetus and placenta, is simple and rapid. However, a highly trained
observer is required for reproducible results, and the lack of a numerical
result for noting trends is a significant disadvantage. The study of
Halperin et al.,23 in which experienced sonographers subjectively assigned
patients to groups with normal, borderline-low, or reduced amniotic fluid
volume, found that more experienced sonographers had significantly higher
intraobserver correlation scores ([kappa] = .94 vs. [kappa] = .63). Despite
the problems with reproducibility, Moore et al.10 showed that welltrained
observers could subjectively identify patients with oligohydramnios with an
intraclass correlation coefficient of 0.81.
Maximum Vertical Pocket (MVP). This technique evolved from the studies of
Chamberlain et al.2 in which the single deepest uninterrupted pocket of
amniotic fluid is measured. The criteria for "abnormal" amniotic fluid
volume based on the MVP are summarized in Table 1. Oligohydramnios was
defined as the absence of any amniotic fluid pocket of at least 1 cm in
depth ("1-cm rule") and polyhydramnios was any pocket >8 cm. This scale,
which has been widely adopted, has the advantages of providing a
semiquantitative result and reasonable
predictive power for poor pregnancy outcome.2 However, although the
single-pocket technique is reasonably reproducible, the criteria for a
"normal" MVP were derived from a group of high risk, predominantly postdate
patients. Moreover,
the predictive power for poor perinatal outcome is limited. Although Hoddick
noted that a
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TABLE 1. Qualitative Amniotic Fluid Assessment Using the Maximum
Vertical Pocket Visible on Ultrasound
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Amniotic Fluid Index (AFI). First described by Phelan et al.,27 this method
involves summing the maximum vertical pockets in each of four quadrants of
the uterus. The original study, conducted on 197 patients from 12-42 weeks
indicated that the mean AFI increased from 7 cm-20 cm until 26 weeks, then
plateaued at approximately 16 cm for the remainder of gestation.
A later study defined cutoffs for the AFI, shown in Table 2.27 Although
derived from an high-risk population, the AFI cutoffs provided a useful
working definition of "normal" amniotic fluid (AFI = 8-18 cm). Patients
with AFI values
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TABLE 2. Diagnostic Categories of the Amniotic Fluid Index
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To more rigorously define "normal" values for the AFI, Moore and Cayle 28
measured the AFI cross-sectionally in 791 patients with uncomplicated
pregnancies from 16-44 weeks. The mean AFI and percentiles throughout
gestation from this group of normal pregnancies are shown in Figure 2. Near
term, the mean AFI is 12 cm; the 95th percentile (polyhydramnnios) is ~20
cm; the 5th percentile
(oligohydramnios) is ~7 cm. Notably, the 5-cm AFI value defined as
oligohydramnios in the earlier studies of postdate patients occurs in only
1% of normal term patients.
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FIG. 2. Amniotic Fluid Index (in mm) plotted with gestational age
(weeks). The solid line denotes the 50th percentile, dashed lines the 5th
and
95th percentiles, and dotted lines +/- 2 standard deviations (2.5th and
97.5th
percentiles). Reproduced with permission from Moore TR, Cayle JE. The
amniotic fluid index in normal human pregnancy. Am J Obstet Gynecol. 1990;
162:1168-1173.
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INTERSERVER AND INTRAOBSERVER RELIABILITY
Reproducibility is a desirable characteristic of an amniotic fluid volume
estimation method. When intraobserver and interobserver variations were
assessed by Moore and Cayle,28 mean errors of 5 mm and 10 mm were noted
respectively, equivalent to 3% and 7% of the AFI. However, although the
absolute error was fairly constant (~1 cm), the percentage error was as
high as 10%-15% with AFIs
Amniotic Fluid Index Vs. Maximum Vertical Pocket. The relative efficiency
of the AFI and MVP has been assessed in multiple studies. Moore's 33
comparison
of the AFI and MVP in 1,168 patients noted a correlation coefficient of
0.51. However, the sensitivity of the MVP technique in identifying
oligohydramnios was poor: 58% of cases with oligohydramnios by AFI had
"normal" values according to the single-pocket technique. The sensitivity
and specificity in detecting polyhydramnios were 42% and 51%, respectively.
Other investigators comparing MVP and AFI have reached similar conclusions
regarding the superiority of four-quadrant method.34-37
Correlation of Sonographic Estimates with Actual Amniotic Fluid Volume.
Several recent studies have addressed the relationship between the AFI and
actual amniotic fluid volume. Strong et al.38 correlated an intrauterine
infusion of 250 ml of saline with a rise in the AFI of 4 cm. Chauhan 5
recorded a mean increase in AFI of 5.8 +/- 2.6 cm after 250 ml saline were
infused into patients with ruptured membranes. These infusion studies
suggest that a near-term mean AFI of 14 cm is equivalent to 700 ml, a value
notably similar to the 717 ml reported by Brace and Wolf.22
Magann et al.39 compared the AFI, MVP, and a two-diameter sonographic
method to amniotic fluid volume calculated from direct measurements of
para-amino
hippurate (PAH) dilution after injection into the amniotic cavity during
amniocentesis in 40 third trimester pregnancies. A PAH mixing time of 30-40
minutes was used, and each US measurement was obtained as a single value.
Oligohydramnios was defined as 1500 ml. "Normal" amniotic fluid volume was
correctly predicted by the AFI in only 50%, the MVP was correctly predicted
in 50%, and the two-diameter method was correctly predicted in 61%. In
detecting
oligohydramnios, AFI was 65% efficient, MVP was 63% efficient, and
two-diameter was 75% efficient (P
A similar study by Dildy et al.,40 performed using PAH in 50
third-trimester women undergoing amniocentesis, showed that the AFI was
highly predictive of actual volume with a correlation coefficient of 0.84,
and a mean error of 7%. An exponential curve relating amniotic fluid volume
(AFV) and AFI was determined (AFV = exp (5.19 + 0.093 - AFI). Using this
formula, a near term AFI of 14 cm would correlate with an AFV of 660 ml,
(90% confidence interval 315-1,383 ml). However, when oligohydramnios was
present, the AFI overestimated the true volume by as much as 89%; with
hydramnios, AFI underestimated actual volume by 54%. Dildy et al. concluded
that current methods of amniotic fluid volume estimation with US are
adequate for identifying grossly abnormal volumes of fluid, but lack the
precision necessary for detailed volumetric correlations. These results are
consistent with those of Moore and Cayle,28 and Croom et al.,34 who noted
that errors were magnified with oligohydramnios.
In summary, these studies show that the AFI is an adequately reproducible
and proportional index of actual amniotic fluid volume. Amniotic fluid
index is probably more reliable in identifying extremes of amniotic fluid
volume than the MVP. However, no existing US method of assessing amniotic
fluid volume has accuracy consistently less than +/- 25%.
TECHNICAL ASPECTS OF PERFORMING THE AMNIOTIC FLUID INDEX
The step-by-step technique for determining the AFI is given in Table 3 and
is diagrammed in Figure 3. Adherence to these guidelines helps minimize
observational variability. As noted by Bruner et al.,29 when serial
assessments of AFI are necessary, repeat examinations by a single observer
results in the best
accuracy. Other factors that could adversely affect AFI reproducibility
include measuring very narrow pockets, measuring into gray or "fuzzy"
tangential sections of placenta or fetal parts, and measuring the same
pocket twice in adjacent quadrants.
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TABLE 3. Amniotic Fluid Index Technique
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FIG. 3. Schematic diagram of technique for measuring the four
quadrant amniotic fluid index (AFI). See Table 3 for explanation.
Reproduced with permission from Gilbert WM. Disorders of Amniotic Fluid. In:
Creasy RK, Resnik R, eds. Maternal Fetal Medicine. 3rd ed. Philadelphia:
W.B. Saunders.
1994:620-621.
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Effect of Fetal Movement. The effect of fetal movement during the scanning
process, shifting amniotic fluid between quadrants, was addressed by Wax et
al.,41 who found that the mean change in the amniotic fluid index after
fetal movement was 1.5 +/- 0.1 cm and 2.5 +/- 0.2 cm for post-movement
determinations by the same examiner and blinded observer, respectively (P
Effect of Transducer Type. Choice of US transducer (linear, sector,
curvilinear) appears to have no significant effect on AFI reproducibility.
When Del Valle et al.42 compared AFI measurements among 3 transducer types
in 65 women between 26 and 40 weeks' gestation, AFI values obtained with
the sector or convex transducers were as reliable as those obtained with
the gold standard linear transducer.
Indications for and Frequency of Amniotic Fluid Volume Assessment
Given the convenience and reproducibility of the AFI, occasions in which
its inclusion in US and fetal biophysical assessments have expanded
markedly. Suggested indications for amniotic fluid volume evaluation are
summarized below.
During Routine Or Targeted Ultrasound Examinations After 16 Weeks. Although
recording the relative amount of amniotic fluid present at a routine
16-week size dates US scan may seem superfluous, the precise AFI value and
associated gestational percentile offers valuable clues to existing
pathology (congenital anomalies). When subsequent amniotic fluid volume
abnormalities are noted, AFI percentiles from an earlier scan can provide
important information about the
time course and possible cause of the derangements.
Monitoring Patients With Preterm Premature Rupture of Membranes. Serial
evaluation of amniotic fluid volume may provide important prognostic
information in patients with ruptured membranes who are being managed
conservatively. Silver et al.43 found an inverse correlation between
amniotic fluid index and latency period (the time interval from membrane
rupture to labor onset and
delivery) in patients with premature rupture of membranes (PROM). Patients
with adequate fluid had a significantly longer interval between PROM and
delivery than with those with oligohydramnios (P P
During Antepartum Testing. Many studies have underscored the value of
adding the AFI to the non-stress test (NST). Rutherford et al.45 reported
that the incidence of abnormal perinatal outcome (e.g., fetal distress,
cesarean section, meconium staining, low Apgar scores) was higher among
patients with AFI 5 cm and a reactive NST may reduce the risk of fetal death
to a negligible level.
The relative value of the amniotic fluid volume measurement within the
fetal biophysical profile was reported by Youssef et al.48 The AFI was more
sensitive in predicting mortality (87.5%), low 5-minute Apgar score (88.8%),
fetal distress during labor (86.6%), meconium-stained amniotic fluid,
(63.6%), and the presence of intrauterine growth retardation (79.4%) than
the fetal biophysical profile score overall. Furthermore, these
investigators showed that using the AFI instead of a single pocket
measurement in the fetal biophysical profile increased the sensitivity and
positive predictive value of the fetal biophysical profile from 64.7%-76.4%
and from 45.8%-68.4%, respectively.
The frequency at which AFI evaluations should be repeated during antepartum
testing (weekly, twice weekly) remains controversial. Marks et al.49
reported a progressive decrease in AFI after 40 weeks of only 25% per week,
suggesting that weekly AFI measurements are adequate. When Lagrew et al.50
studied the change in AFI during twice weekly testing, patients with
borderline AFIs (5-8
cm) had a 5% chance of oligohydramnios developing within the next 4 days,
whereas patients with normal AFIs (>=8 cm) had only a 0.54% risk. Patients
with oligohydramnios (AFI 50% decrease) had no association with adverse
fetal outcome provided the final value remained >5.0 cm.
These results suggest that AFI can be repeated weekly if >8 cm. When AFI
falls below 8 cm but is >5 cm, evaluations should be performed twice weekly.
For clinical convenience, the authors performs the AFI in conjunction with
each antepartum biophysical test, even if performed twice or three times
weekly. With regard to intervention, the high incidence of meconium
staining and fetal distress among patients with AFI
Summary
Amniotic fluid volume estimation has become an integral part of fetal
evaluation. Although the sonographic techniques clinically available are
limited in their accuracy and predictive value, the careful performance of
AFI measurements provides important and complementary clinical data on
which to base management decisions in pregnancies at risk.
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