Assessment of amniotic fluid volume

INTRODUCTION — Qualitative or semiquantitative assessment of amniotic fluid volume (AFV) is a standard component of every obstetric ultrasound examination, regardless of indication, because it is an indicator of fetoplacental health and thus useful for identifying, monitoring, and managing complicated pregnancies [1,2].

The sonographic assessment of AFV will be discussed here. Factors that influence AFV and specific disorders of AFV (oligohydramnios and polyhydramnios) are reviewed separately:

●(See "Physiology of amniotic fluid volume regulation".)

●(See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations".)

●(See "Polyhydramnios: Etiology, diagnosis, and management in singleton gestations".)

CLINICAL SIGNIFICANCE — Normal AFV is a sign of fetoplacental health.

Abnormal AFV is a marker for (and may reflect the severity of) a variety of pregnancy complications, including prelabor rupture of membranes, some congenital anomalies (particularly involving the gastrointestinal or urinary tracts), placental insufficiency (particularly fetal growth restriction or postterm dysmaturity), twin-twin transfusion syndrome, and aneuploidy (particularly trisomy 18). Abnormal AFV can adversely affect pregnancy outcome, independent of its cause because it increases the risks of compression or prolapse of the umbilical cord; fetal deformation, malpresentation, or demise; and postpartum uterine atony [3-5].

For these reasons, ultrasound assessment of AFV is used in conjunction with other clinical and sonographic assessments to evaluate fetal well-being and manage complicated pregnancies. For example, in the late second and third trimesters, AFV assessment is one of the components of the biophysical profile (table 1) and the modified biophysical profile (nonstress test plus ultrasound assessment of AFV) for antepartum fetal surveillance [6,7]. In pregnancies complicated by fetal growth restriction, assessment of AFV, fetal weight, and umbilical artery Doppler are used in decision-making regarding timing of delivery. (See "Overview of antepartum fetal assessment" and "Fetal growth restriction: Pregnancy management and outcome".)

AFV REFERENCE STANDARDS — Dye dilution techniques and collection of amniotic fluid at cesarean birth have been used to establish reference standards for normal AFV in research studies [8,9]. Normal AFV, as measured by these validated methods, varies across gestation (figure 1). Dye dilution is invasive, cumbersome, and requires specialized technical skills and laboratory support, while collection of amniotic fluid during labor or at delivery is not an option for ongoing fetal assessment during pregnancy since full collection is challenging and the volume of blood contamination must be calculated and subtracted from the total volume of fluid collected to determine the actual AFV.

Ultrasound examination is the only practical clinical method to assess AFV. Both qualitative and semiquantitative techniques identify normal volumes but perform less well in identifying low and high AFV when compared with dye-dilution techniques [10]. Sonographic reference standards for assessment of AFV are described below. (See 'Sonographic assessment of AFV' below.)

SONOGRAPHIC ASSESSMENT OF AFV — AFV can be assessed sonographically using a qualitative or semiquantitative approach; each has limitations in the detection of abnormal AFV. Regardless of the approach, the best thresholds for identifying and managing pregnancies with increased risk of fetal or neonatal morbidity are uncertain [11]. (See 'Qualitative assessment' below and 'Semiquantitative techniques' below.)

Our approach — We perform a qualitative assessment of AFV in all pregnancies undergoing ultrasound examination. We also perform a semiquantitative measurement report using both the single deepest pocket (SDP) and the four-quadrant amniotic fluid index (AFI) in all third-trimester ultrasound examinations and in those second-trimester examinations in which the qualitative assessment of AFV is abnormal, the pregnancy is at high risk for adverse pregnancy outcome, or a biophysical profile or modified biophysical profile is being performed. Although either SDP or AFI are appropriate clinical assessments, we typically measure both, particularly with pathologic cases. If the AFV is low then we use the SDP as the criteria for oligohydramnios, whereas if the AFV is high we use the AFI criteria for polyhydramnios.

Both the SDP and AFI techniques are commonly used worldwide, and neither technique is clearly superior. Results from a large trial (>500 pregnancies) showed that both AFI and SDP identify normal AFV with sensitivity >90 percent, but both techniques overdiagnose AFV abnormalities and may lead to unnecessary intervention [12]. Using the SDP overly diagnoses polyhydramnios, while the four-quadrant AFI overdiagnoses oligohydramnios. In a systematic review of randomized trials comparing the SDP and AFI, use of the AFI did not improve peripartum outcomes compared with the SDP, although oligohydramnios was diagnosed more frequently (relative risk [RR] 2.3) and induction of labor (RR 2.1) and cesarean birth for fetal distress occurred more frequently (RR 1.5) [13]; a subsequent trial affirmed this finding [14].  

Qualitative assessment — Qualitative assessment of AFV refers to the subjective assessment of AFV by the sonographer, without sonographic measurements [10]. The amniotic fluid is imaged and subsequently reported as oligohydramnios, normal, or polyhydramnios based on the sonographer's clinical expertise.

●Accuracy – In a study of 63 pregnancies with dye-dilution-determined AFV as the reference standard, qualitative assessment of AFV by an experienced examiner had similar accuracy as semiquantitative techniques (AFI, SDP, and two-diameter pocket) [10]. The accuracy of sonographic assessment ranged between 60 and 70 percent.

Semiquantitative techniques

Single deepest pocket — The SDP (also called the maximum vertical pocket [MVP], deepest vertical pocket [DVP], or single deepest vertical pocket [SDVP]) is the vertical dimension in centimeters of the largest pocket of amniotic fluid not persistently containing fetal extremities or umbilical cord (on gray-scale examination).

We use the following thresholds, developed by a 2014 consensus panel at a fetal imaging workshop [15]:

●Oligohydramnios – Depth <2 cm

●Normal – Depth ≥2 cm and <8 cm

●Polyhydramnios – Depth ≥8 cm

The width of the pocket must be at least 1 cm. In patients with a persistent single loop of cord in the SDP, the most accurate technique is to measure the largest vertical distance to the cord, either above or below but not through the cord [16].

These thresholds continue to be generally accepted, although slight variations are common (eg, <2 versus ≤2 to define oligohydramnios, >8 versus ≥8 cm to define polyhydramnios) [17]. We believe the horizontal component of the vertical dimension should be at least 1 cm wide, although some sonologists consider 2 cm the minimum horizontal threshold. Requiring a 1 cm minimum width for the SDP improves sensitivity for detecting oligohydramnios and predicting an adverse outcome [18,19].

Although we consider an SDP ≥2 and <8 cm normal and manage the pregnancy accordingly, data to guide management of pregnancies with SDP between 2 and 3 cm are limited. As with other borderline laboratory values, the clinician may elect closer follow-up (eg, twice weekly assessments) of patients with SDP values that approach the level of oligohydramnios or with SDP values derived from only a single visible pocket (in such cases, a simultaneously performed AFI may be ≤5 cm).

●Accuracy — The SDP detects fewer pregnancies with low AFV than dye dilution and direct assessment methods.

•In a study of 40 pregnancies that compared the SDP with dye-determined AFV, 94 percent of pregnancies with normal dye-determined AFV were identified by SDP, but none of the pregnancies with a low dye-determined AFV were detected (low dye dilution AFV was defined as ≤5^th percentile) [20].

•In another study of 45 pregnancies at term that compared SDP with direct measurement of AFV at cesarean birth, SDP identified only 18 percent of the pregnancies with low AFV (<200 mL) identified by collection of amniotic fluid [21].

Amniotic fluid index — The AFI is calculated by dividing the uterus into four quadrants using the linea nigra for the right and left divisions and the umbilicus for the upper and lower divisions. The maximal vertical amniotic fluid pocket diameter in each quadrant not containing fetal extremities or cord (on grayscale examination) is measured in centimeters; the sum of these measurements is the AFI. A 2014 consensus panel at a fetal imaging workshop suggested the following interpretation of AFI [15]:

●Oligohydramnios – AFI ≤5 cm

●Normal – AFI >5 cm and <24 cm

●Polyhydramnios – AFI ≥24 cm (table 2)

These thresholds continue to be generally accepted, although small variations are common. For example, polyhydramnios has been defined as AFI >18, >20, >24, and >25 cm [22-24]. Gestational age, which is not considered in interpretation, also has an effect: in the third trimester, an AFI of 24 is generally above the 95^th percentile and an AFI of 5 cm is generally below the 2.5^th percentile [25-27]. Gestational age is not considered, in part, because AFI criteria have been established with a greater focus on adverse outcomes [28] than distribution across gestation.

An AFI of 5.1 to 8 cm has been termed borderline, but the clinical implications are unclear, and no high-quality evidence exists to support additional antenatal assessment of these pregnancies [29]. In these borderline AFI cases, use of the SDP to determine surveillance and/or intervention is preferable. We manage pregnancies with an AFI of 5.1 to 8 cm the same way as an AFI above 8 cm, without additional testing or hydration. However, it is also reasonable for the clinician to elect closer follow-up (eg, twice weekly AFV assessments) of patients with AFI values that approach the level of oligohydramnios.

●Accuracy – The accuracy and prognostic value of the AFI has been examined in several studies, which have shown that an abnormal AFI (oligohydramnios or polyhydramnios) is neither highly accurate nor highly predictive of adverse outcome [3,8,20,30-32]. Many pregnancies with normal AFV are falsely characterized as abnormal, and a large number with truly abnormal AFVs will be missed. Use of percentiles rather than fixed cutoffs to identify low or high AFVs does not improve the accuracy of the method.

•In four studies that compared AFI with dye-dilution-determined AFV, the two techniques were concordant in 70 to >90 percent of pregnancies with normal AFV by the reference standard [8,12,20,30]. AFI was less accurate with abnormal AFVs:

-In one of these studies, at low AFV, the AFI overestimated dye-determined volumes by 89 percent, while at high AFV, the AFI underestimated dye-determined volumes by 54 percent [8].

-In two of the studies, AFI identified only approximately 10 percent of pregnancies with low AFV by dye dilution [20,30].

•In an alternate analysis, dye-dilution-determined AFV was used to evaluate 291 singleton pregnancies with AFI and SDP <3^rd and 5^th percentiles and >95^th and 97^th percentiles, adjusted for gestational age [32]. The sensitivity of AFI or SDP <3^rd and 5^th percentiles to detect oligohydramnios ranged from 11 to 27 percent versus 33 to 46 percent for detecting polyhydramnios. Use of percentiles was no better than fixed cutoffs (eg, AFI ≤5 or >25, SDP <2 or >8 cm) for detecting oligohydramnios and polyhydramnios.

Technical considerations

Transducer position — When measuring fluid pockets for the AFI and SDP, most older studies have recommended holding the transducer perpendicular to the floor while some have recommended holding it perpendicular to the uterine contour [17,22]. More recent studies suggest that either transducer position is acceptable [33,34]. Regardless of the position used, the operator should be careful to avoid angling the transducer at the uterine periphery as a thin rim of amniotic fluid may be misdiagnosed as a pocket.

Use of concurrent color Doppler — The concurrent use of color Doppler has been proposed to identify umbilical cord in an amniotic fluid pocket that is not seen on gray-scale ultrasonography alone, given that pockets of amniotic fluid containing umbilical cord should not be used for measurement of AFV. Although used by some sonologists, we caution against using color Doppler as a component of AFV assessment because the current sonographic standards for assessment of normal versus abnormal AFV were established without use of color Doppler and its use results in overdiagnosis of oligohydramnios [35-37].

Our reasoning for avoidance of concurrent color Doppler is supported by a study of AFV assessment with and without concurrent color Doppler in which using Doppler did not improve the diagnostic accuracy of oligohydramnios-based dye-dilution techniques and misclassified 9 of 42 pregnancies with normal amniotic fluid as having oligohydramnios [37]. Both AFI and SDP measurements were reduced by approximately 20 percent when color Doppler was used. In addition, a subsequent study of 428 patients comparing pregnancy outcomes when the AFV was classified as low by color Doppler and normal by gray-scale ultrasound (group 1) versus low by both color Doppler and gray scale (group 2) found that both groups had similar composite perinatal complications, mode of delivery, and composite neonatal outcomes [38]. The use of color Doppler did not increase the detection of adverse intrapartum or perinatal outcomes, thus supporting our approach.

SPECIAL POPULATIONS

Gestational age 14 to 20 weeks — We suggest using the SDP technique to estimate AFV in pregnancies between 14 and 20 weeks. AFI reference ranges (table 3), SDP reference ranges (table 4), and two-dimensional pocket reference ranges (table 5) exist for normal pregnancies in this gestational age range [39,40]. However, the normal AFV at any point in gestation sometimes differs depending on the population being investigated; thus, the percentiles in these tables may not apply to patients in other clinical settings.

There is limited information on the earliest gestational age that ultrasound measurements can be used to estimate AFV. One problem with using the amniotic fluid index (AFI) this early in gestation is that only two quadrants exist before 20 weeks (the fundus may be at or below the umbilicus). In one study of AFI in pregnancies from 16 to 44 weeks, this problem was addressed by dividing the uterus into four quadrants created by an imaginary vertical line along the maternal midline and another imaginary transverse line midway between the top of the uterine fundus and the symphysis pubis [39]. In another study of AFV in pregnancies from 14 to 41 weeks, AFV was estimated using the AFI, single deepest pocket (SDP), and two-diameter pocket techniques, but the AFI was calculated from the sum of the SDP of only two quadrants in those pregnancies in which the uterine size was below the umbilicus [40].

The correlation between SDP and dye-determined AFV in early pregnancies is uncertain. The only study that correlated ultrasound measurements with dye-determined AFV in 42 singleton pregnancies between 15 and 23 weeks found that the SDP correctly identified 3 out of 8 with dye-determined oligohydramnios and 26 out of 29 with normal dye-determined AFV compared with 2 out of 8 with oligohydramnios and 22 out of 29 with normal fluid volume using the AFI [41].

Multifetal pregnancy — We suggest qualitative assessment of AFV in multifetal pregnancies. If a semiquantitative measurement is needed, we measure the SDP of each amniotic sac.

In twin pregnancies, the assessment of AFV is an important part of their overall evaluation since they have a perinatal mortality rate severalfold higher than singleton pregnancies. As with singleton pregnancies, normal AFV in diamniotic twin pregnancies has been determined by studies using dye-dilution techniques [42]. These studies have noted that (1) the mean AFV in each sac of a twin pregnancy is slightly higher than that of a singleton pregnancy of the same gestational age, and (2) qualitative and semiquantitative sonographic techniques tend to underestimate abnormalities of AFV compared with dye-dilution techniques [43].

Three ultrasound techniques have been used to estimate the AFV in diamniotic twin pregnancies. All three methods correlate poorly with dye-determined low or high volumes, in part because the position of the dividing membrane affects the interpretation if each twin is evaluated separately. The limited available evidence is reviewed below.

●Qualitative assessment – A study including 23 twin pregnancies subjectively and objectively evaluated AFV and then compared these results with the dye-determined volume [44]. There were no differences in the accuracy of the subjective versus the objective evaluation of AFV to identify abnormal volumes, and both techniques were equally poor in the identification of the extremes (high and low) of AFV.

●SDP – The SDP of each sac is measured and the results interpreted using the SDP criteria described above for singletons [43] (see 'Single deepest pocket' above). This is possible because the 2.5^th and 97.5^th percentiles for twins are 2.3 cm and 7.6 cm, respectively, which are similar to the singleton cutoffs of 2 cm and 8 cm to define oligohydramnios and polyhydramnios [15]. Using this technique, high SDPs have been correlated with abnormal fetal heart rate tracings in labor and an increased frequency of cesarean deliveries for fetal intolerance of labor [45].

●AFI – Although an AFI in twins can be performed using the same technique as in singleton pregnancies (see 'Amniotic fluid index' above), inaccurate assessments occur if the membrane separating diamniotic twins is not considered. In a study using the summated twin AFI (four-quadrant assessment) and dye-determined AFV, 94 percent of the twins were identified as normal by summated AFI, whereas only 52 percent had normal volumes by dye dilution. Summated AFI had only a 13 percent sensitivity in predicting abnormal AFVs [46].

To improve the performance of AFI in twin pregnancies, one study located the dividing membrane and measured the AFI in four quadrants of each sac [47]; however, this approach has not been validated.

MAGNETIC RESONANCE IMAGING — Magnetic resonance imaging (MRI) is neither a practical nor economical method for assessment of AFV. However, if MRI is performed for assessment of fetal anatomy, then AFV can also be assessed. Software has been developed for this purpose or AFV can be calculated manually using a hand-tracing method [48-50].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Oligohydramnios and polyhydramnios".)

SUMMARY AND RECOMMENDATIONS

●Utility – Ultrasound estimation of amniotic fluid volume (AFV) is used in conjunction with other clinical and sonographic assessments (eg, biophysical profile, nonstress test, estimated fetal weight, anatomic survey) to provide information for assessing fetal well-being and identifying and managing complicated pregnancies. (See 'Clinical significance' above.)

●Candidates and approaches – A qualitative assessment of AFV is routinely performed in all pregnancies undergoing ultrasound examination. In addition, we perform semiquantitative measurements in (see 'Our approach' above):

•All third-trimester ultrasound examinations

•All second-trimester ultrasound examinations that have an abnormal qualitative assessment OR the pregnancy is at high-risk for adverse pregnancy outcome OR a biophysical profile or modified biophysical profile is being performed.

●Techniques – Both the single deepest pocket (SDP) and amniotic fluid index (AFI) semiquantitative techniques are commonly used worldwide. Neither technique is clearly superior. We typically measure both. If the AFV is low, then we use the SDP as the criteria for oligohydramnios, whereas if the AFV is high we use the AFI criteria for polyhydramnios. We also prefer SDP in pregnancies between 14 and 20 weeks. (See 'Single deepest pocket' above and 'Amniotic fluid index' above and 'Gestational age 14 to 20 weeks' above.)

For both techniques, the operator should be careful to avoid angling the transducer at the uterine periphery as a thin rim of amniotic fluid may be misdiagnosed as a pocket. (See 'Technical considerations' above.)

●Accuracy – Qualitative and semiquantitative ultrasound techniques perform well in identifying pregnancies with normal AFVs but are less accurate than dye dilution and amniotic fluid collection techniques for diagnosing oligohydramnios and polyhydramnios. The SDP overly diagnoses polyhydramnios, while the AFI overly diagnoses oligohydramnios. (See 'Sonographic assessment of AFV' above.)

●Multifetal pregnancies – Ultrasound estimates of AFV in multifetal pregnancies correlate poorly with dye-determined low or high volumes. We suggest qualitative assessment in multifetal pregnancies and determine the SDP for each amniotic sac if a semiquantitative measurement is needed. The results for each fetus are interpreted using the same SDP criteria used for singletons. (See 'Multifetal pregnancy' above.)