The evaluation of amniotic fluid volume (AFV) is an established part of the antenatal surveillance of pregnancies at risk for an adverse pregnancy outcome. The two most commonly used ultrasound techniques to estimate AFV are the amniotic fluid index (AFI) and the single deepest pocket (SDP). Four studies have defined normal AFVs, and although their normal volumes have similarities, there are also differences primarily due to the statistical methodology used in each study. Dye-determined AFV correlates with ultrasound estimates for normal fluid volumes but correlates poorly for oligohydramnios and polyhydramnios. The addition of color Doppler in estimating AFV leads to the overdiagnosis of oligohydramnios. Neither the AFI nor the SDP is superior in identifying oligohydramnios, but the SDP is a better measurement choice as the use of AFI increases the diagnosis rate of oligohydramnios and labor inductions without an improvement in pregnancy outcomes.
Highlights
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Amniotic fluid volume assessment is essential in high-risk pregnancies.
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Ultrasound accurately estimates normal amniotic fluid volume but not oligohydramnios or polyhydramnios.
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Amniotic fluid index overdiagnoses oligohydramnios leading to more labor inductions without improving perinatal outcome.
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Color Doppler does not improve the accuracy of identifying actual oligohydramnios.
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Single deepest pocket is a better measurement than the amniotic fluid index in diagnosing oligohydramnios.
Introduction
The assessment of amniotic fluid volume (AFV) is an established part of the antenatal fetal surveillance of a pregnancy at risk for an adverse pregnancy outcome. The gold standard for antenatal surveillance for many US-based investigators is the contraction stress test (CST), which has a false negative rate of .4/1000. However, because of the necessity to have either spontaneous contractions or to start an intravenous line and induce contractions with oxytocin and the high false positive results, this test is less frequently used today . Currently, two commonly used antenatal tests are the BPP (biophysical profile) and the modified BPP. The components of the BPP include non-stress test or cardiotocography, fetal breathing, fetal movement, fetal tone, and AFV assessment . The components of the modified BPP include non-stress test or cardiotocography and AFV assessment . Both tests estimate the AFV by ultrasound as an essential component of the antenatal test. Both tests have good reliability with a false negative rate of .6/1000 for the BPP and a false negative rate of .8/1000 for the modified BPP . The significance of estimating the AFV is underscored with the modified BPP. The false negative rate is observed to increase to 1.9–5/1000 when the non-stress test/cardiotocography is done as a stand-alone test without an estimation of the AFV ( Table 1 ). The CST can identify both acute and chronic stress by the presence of repetitive late decelerations, whereas the BPP can only identify acute stress. The modified BPP is able to identify both acute and chronic stress by the addition of the AFV measurement. It is believed that the fetus under chronic stress will shunt blood to the brain, heart, and adrenal glands at the expense of other bodily organs. This shunting of blood away from the kidneys results in decreased renal perfusion and, consequently, to decreased fetal urinary output. This ultimately causes a decrease in the AFV. Therefore, the estimation of AFV by ultrasound is believed to aid in the recognition of the fetus under chronic stress that results from uteroplacental insufficiency.
| Test | False Negative Rate |
|---|---|
| Contraction Stress Test | .4/1000 |
| Biophysical Profile | .6//1000 |
| CTG and AFV Estimate | .8/1000 |
| CTG only | 1.9–5/1000 |
The Pathophysiology of Amniotic Fluid
Amniotic fluid provides a supportive and protective environment for fetal development during gestation by protecting the fetus from trauma; it possesses bacteriostatic properties, allows fetal movements and thus fosters the development of the limbs and lungs and prevents compression of the umbilical cord . The dynamics of the regulation of AFV are complex, with the entire volume of AF turning over on a daily basis . The production and resorption of AFV is influenced by several sources, including fetal urination; fetal swallowing; fluid production by the fetal lung, skin and oral-nasal cavities; the intramembranous pathway; and the transmembranous pathway . Fetal urine production is the main contributor to AFV in the second half of pregnancy .
The Pathophysiology of Amniotic Fluid
Amniotic fluid provides a supportive and protective environment for fetal development during gestation by protecting the fetus from trauma; it possesses bacteriostatic properties, allows fetal movements and thus fosters the development of the limbs and lungs and prevents compression of the umbilical cord . The dynamics of the regulation of AFV are complex, with the entire volume of AF turning over on a daily basis . The production and resorption of AFV is influenced by several sources, including fetal urination; fetal swallowing; fluid production by the fetal lung, skin and oral-nasal cavities; the intramembranous pathway; and the transmembranous pathway . Fetal urine production is the main contributor to AFV in the second half of pregnancy .
What Is Normal Amniotic Fluid Volume?
It is important to clearly define what a normal AFV is in order to then identify what is abnormal. The definition is an important one because it impacts pregnancy management. Multiple studies have shown that low abnormalities of AFV (oligohydramnios) have been associated with adverse pregnancy outcomes . Multiple definitions of what constitutes an abnormal AFV can be found in the literature . Oligohydramnios, or low AFV, has been defined as less than 200 mL , less than 500 mL total volume , less than the 5 th percentile for gestational age , as SDP less than 2 cm , an AFI of less than 5 cm , or an AFV that is subjectively low . Polyhydramnios, or an increased AFV, has been defined as a total volume that is greater than 2000 mL , an AFV that is greater than the 95 th percentile for gestational age , an SDP of greater than 8 cm , an AFI of greater than 24 cm or greater than 25 cm , or an AFV that is subjectively increased .
There are 4 studies that have defined normal AFV across gestation. Brace et al. derived from the literature normal volumes using dye-dilution techniques and direct measurements at the time of hysterotomy of 705 pregnancies without fetal anomalies, maternal or fetal disease, fetal death, or spontaneous abortion . The investigators observed that the AFV did not change significantly between 22 and 39 weeks. The AFV peaked between 33 and 34 weeks with a subsequent decline thereafter. Magann et al. evaluated 144 normal pregnancies in a singleton institution using dye-dilution techniques to create a growth curve of normal pregnancies across gestation. The investigators observed that the AFV continued to increase during gestation, peaking at 40 weeks . Queenan et al. also used dye-dilution techniques to assess 172 patients between 15 and 42 weeks and observed a peak of AFV at 33 to 34 weeks with a decline thereafter . Sandlin et al. evaluated 379 normal pregnancies from 16 to 41 weeks (144 of these pregnancies had been previously reported by Magann ) using both dye-dilution techniques and direct measurement at the time of cesarean section and used modeling by quantile regression to create a normative chart of AFV across gestation with a peak at 38 weeks . There are similarities but also differences in their normal volumes across gestation primarily due to the statistical methods used ( Figure 1 ). As evidenced by these studies, there is not yet a clearly defined value for a “normal” AFV. What should define normal is a value that is neither excessively high nor low; what should drive the definition of abnormal is poor clinical outcomes associated with such measurements.
Correlation of Dye-determined or Directly Measured AFV to Ultrasound Estimates of Amniotic Fluid Volume
There have been a number of investigations that have correlated the ultrasound estimate of AFV to a true volume of amniotic fluid (dye-determined or directly measured at the time of cesarean section). In a study of 5 near-term sheep presented as an abstract at the Society of Gynecological Investigation in 1988, the amniotic and allantoic fluids were drained from the sheep and the uteri were filled with 2 to 2.5 liters of normal saline. At each 100 mL infusion of saline, an AFI was done. Using curve fitting formulas, there was a good correlation between the increasing AFI and the amount of saline infused (r = 0.94). Unfortunately, in a study by Sepulveda in 16 pregnancies with mid-gestation oligohydramnios, the correlation of the infused saline with an increasing AFI was not as good . These women underwent amniocentesis with infusions of saline and ultrasound estimates of AFV using the SDP and the AFI. It was observed that only 30% of the variation of the ultrasounds could be explained by the saline infusion. Croom et al. correlated AFI and SDP with dye-determined volumes of 50 women undergoing cesarean deliveries and found good correlation, but only 2 pregnancies had oligohydramnios and none had polyhydramnios . Dildy et al., correlating the ultrasound measurements with dye-determined fluid AFV, observed that the AFI overestimated AFV by 89% at low volumes and by 54% at high volumes . Magann et al. in 2 studies showed that the correlation of the AFI with normal fluid was good, but when the AFV was low, the sensitivity of the AFI to detect oligohydramnios was only 6.7% in one study and 8.7% in the second study . Horsager et al. observed, in a study that correlated ultrasound estimate with directly measured AFV at the time of cesarean delivery, that the sensitivity of the AFI to detect directly measured oligohydramnios was only 18% . Magann, in a study of the SDP to detect oligohydramnios, observed that only 14% of the dye-determined oligohydramnios pregnancies were identified by using the SDP technique . In the largest study correlating the SDP and AFI with dye-determined AFV, both the AFI (sensitivity 10% and specificity 96%) and the SDP (sensitivity 5% and specificity 98%) were unreliable in identifying oligohydramnios . These studies consistently document that the ultrasound estimate of AFV correlates well with normal dye-determined or directly measured AFVs but poorly with true volumes of abnormal fluid volumes (oligohydramnios and polyhydramnios). Magann et al. has also shown that the accuracy of determining normal AFV was not improved by combining commonly used US techniques including AFI, SDP, 2-diameter pocket technique, and subjective assessment . It was also noted that a simple subjective assessment of AFV by a trained sonographer appears to be just as accurate as the more complex ultrasonic measurements of determining abnormal AFVs ( Table 2 ).
| Study | Measurement AFI/SDP | Sensitivity to detect oligohydramnios |
|---|---|---|
| Magann 33 | AFI | 6.7% |
| Magann 34 | AFI | 8.7% |
| Horsager 35 | AFI | 18% |
| Magann 36 | SDP | 14% |
| Magann 37 | AFI/SDP | 10%/5% |
Two investigations have compared the normative datasets of Brace et al. and Magann et al. in the detection of low, normal, and polyhydramnios using fixed cutoffs stratified by gestational age and in the correlation of AFI and the SDP to the modeled growth curves. In the first study, the fixed cutoffs of AFV of < 500 mL was classified as oligohydramnios, 500–2000 mL as normal AFV, and > 2000 mL as polyhydramnios . The study found that Brace and Magann databases classified the AFVs differently 24% of the time. The Brace dataset classified 19% of the volumes as oligohydramnios, 71% as normal AFV, and 10% as polyhydramnios. The Magann dataset classified 3% as oligohydramnios, 83% as normal AFV, and 14% as having polyhydramnios. The authors concluded that the two datasets were not exchangeable and that abnormal volumes cannot be clearly identified until normal volumes are defined. An investigation that assessed the correlation between the AFI and SDP techniques with the AFV as modeled by normal curves of Brace and Magann revealed that, overall, there was a good correlation between the AFI and SDP and the actual volumes determined by both Brace and Magann . The AFI normal ranges of 5.1– 20 versus 5.1–24 was better correlated with the normal volumes of both Brace and Magann. The AFI and the SDP better predict oligohydramnios using the Brace model, whereas the Magann model more accurately predicts normal AFV and polyhydramnios. The authors suggested that future investigations would need to focus on emerging statistical methodologies to create normal curves that will be better able to define abnormal AFVs.
Estimate of AFV Using Ultrasound
While the measurement of AFV using dye-dilution techniques or direct measurement at the time of cesarean delivery is the most accurate approach to determine the actual AFV, these methods are invasive, labor-intensive, and the measurement can only be performed at the time of cesarean delivery. Therefore, AFV is frequently estimated using ultrasonography . The two techniques most frequently used to measure AFV are the AFI and the SDP. The AFI is calculated by the operator first dividing the uterine cavity into four quadrants, and then measuring the fluid pocket with the largest vertical diameter within each quadrant. Each pocket must have a horizontal diameter of at least 1 cm, and the operator must keep the transducer perpendicular to the floor and not the uterine contour. Measurement of a pocket in which there is the brief appearance of the umbilical cord is acceptable, but if it remains in the pocket, then another pocket should be measured. The sum of these four measurements is then calculated to determine the AFI . An AFI of < 5 is defined as oligohydramnios, 5–24 or 25 as normal, and > 24 or 25 as polyhydramnios. The SDP is calculated by measuring the vertical dimension of the largest pocket of amniotic fluid that does not contain umbilical cord or fetal extremities with a horizontal measurement of at least 1 cm. It is also important that the SDP is measured at a right angle to the floor and not the uterine contour. An SDP of < 2 is classified as oligohydramnios, 2–8 as normal, and >8 as polyhydramnios .
Should the Ultrasound Estimate of AFV Be Performed with Color Doppler or Gray Scale?
The initial assessments to estimate the AFV, the SDP of the BPP, and the AFI of the modified BPP were done using gray scale assessment. If there were multiple loops of cord, or if there were small parts in the pocket, then that pocket of fluid was not measured. Color Doppler has recently been used to assist in the identification of amniotic fluid pockets containing umbilical cord that may not be seen with gray scale imaging alone. It was felt important to identify pockets with umbilical cord in them that may not have been seen with gray scale imaging alone because those pockets would have been excluded from measurement based on the original methods using gray scale alone. The better detection of pockets containing umbilical cord using color Doppler, it was thought, would be a better identifier of oligohydramnios ( Figure 2 ). In a study by Bianco et al., the use of color Doppler compared to gray scale significantly decreased the AFI and increased the number of pregnancies classified as having oligohydramnios . In another study by Goldkrand et al., the investigators also observed that the AFI was less with color Doppler compared to gray scale . Magann et al. observed that the AFI was reduced by approximately 20% when color Doppler was compared to gray scale . The investigators evaluated 67 women to determine if the use of color Doppler increased the detection of dye-determined oligohydramnios, normal AFV, and polyhydramnios. The use of color Doppler compared to gray scale increased the proportion of women classified as having oligohydramnios. However, the use of color Doppler did not accurately identify any more women with actual dye-determined oligohydramnios, but instead labeled 9 additional women with normal dye-determined AFV as having oligohydramnios. The authors concluded that the use of color Doppler leads to the overdiagnosis of oligohydramnios .
