The accuracy of the summated amniotic fluid index in evaluating amniotic fluid volume in twin pregnancies

A systematic review of amniotic fluid assessments in twin pregnancies

The objectives of this systematic review were to examine the reproducibility of sonographic estimates of amniotic fluid volume (AFV) in twin pregnancies, compare the association of sonographic estimates of AFV with dye-determined AFV, and correlate AFV with antepartum, intrapartum, and perinatal outcomes in twin pregnancies. Studies were included if they were adequately powered and investigated antepartum, intrapartum, and/or perinatal adverse outcome parameters in twin gestations. Studies with comparable populations and exclusion criteria were merged into forest plots. Data comparing the accuracy of AFV assessment, correlation of AFV with gestational age, and adverse outcomes were tabulated. Five of the 6 studies investigating AFV by the amniotic fluid index as a function of gestational age reported data fitting a quadratic equation, with fluid volumes peaking at mid gestation and then declining. This trend was less pronounced when AFV was assessed by the single deepest pocket (2 of 4 studies reporting a quadratic fit). Polyhydramnios was associated with prematurity in 2 of 4 studies (1 amniotic fluid index and 1 single deepest pocket), and oligohydramnios was associated with prematurity in 1 single deepest pocket study. Stillbirth was the only intrapartum outcome reported in more than 1 study. Perinatal outcomes associated with polyhydramnios included neonatal death (P < .05 in 1 of 2 studies), low Apgar scores (1 of 2 studies), neonatal intensive care unit admission (1 of 2 studies), and low birth weight (2 of 3 studies).

Accuracy of Sonographic Amniotic Fluid Volume Assessments in Diamniotic Dichorionic Twin Pregnancies: A Literature Review

This literature review is meant to bring to light the controversy over the most accurate method to assess the amniotic fluid volumes (AFV) in diamniotic-dichorionic (di-di) twin gestations. It is hoped that this may initiate consideration of a standardized method so that all sonographers and interpreting physicians will consistently use the same method. There is no definitive standard method for sonographers to obtain an accurate amount of amniotic fluid. Commonly, sonographers and interpreting physicians agree that the most practical and accurate methods for sonographers to use are the amniotic fluid index, largest vertical pocket, and the two-diameter pocket methods. Although these measurements are fast, noninvasive, and relatively consistent in evaluating the relative amount of AFV, only the dye-determined technique is 100% accurate, especially when the AFV is outside the subjective normal ranges and either oligohydramnios or polyhydramnios is present.

The ultrasound estimation of amniotic fluid volume in diamniotic twin pregnancies and prediction of peripartum outcomes

OBJECTIVE

To determine ultrasonic estimations of amniotic fluid in diamniotic twins across gestation using the single deepest pocket (SDP) technique and correlate volumes to intrapartum and neonatal outcomes.

STUDY DESIGN

Prospective observational study using diamniotic dichorionic and diamniotic monochorionic pregnancies. Intrapartum and neonatal outcomes were evaluated.

RESULTS

Between April 2004 and April 2006, 299 women were recruited. There was no significant relationship between SDP and gestational age. Statistically significant differences by SDP groups (for low, normal, and high volumes) include preterm delivery (P = .043; 56, 62, and 86%), abnormal labor tracing (P = .013; 5, 8, and 27%), fetal labor intolerance (P = .026; 6, 9, and 27%), Apgar scores at 5 minutes (P = .028; 17, 10, and 27%) and any neonatal complications (P = .002; 28, 21, and 55%).

CONCLUSION

The SDP is constant between 17 and 37 weeks. Fetal labor intolerance and adverse neonatal outcomes are greater in a twin with hydramnios.

Dye-dilution techniques using aminohippurate sodium: do they accurately reflect amniotic fluid volume?

OBJECTIVE

To determine whether the dye-dilution technique using aminohippurate sodium accurately measures amniotic fluid volume.

METHODS

Singleton pregnancies with intact membranes undergoing a Cesarean delivery had their amniotic fluid volume assessed by the dye-dilution technique and direct measurement.

RESULTS

Fifteen women were prospectively assessed. Six patients had their amniocentesis on the delivery table and nine patients at 4-24 h prior to the Cesarean delivery. The six women undergoing an amniocentesis just before delivery had good concordance between the dye-determined and direct measurement of amniotic fluid volume (r = 0.99, p = < 0.001). Among the nine women with varying times from amniocentesis to direct measurement, the correlation was not significant (r = 0.36, p = 0.08). The percentage difference between the dye-determined and directly measured amniotic fluid volume was significantly smaller in the women undergoing amniocentesis just prior to delivery (7%) than in the women with varying times from amniocentesis to delivery (37%, P < 0.001).

CONCLUSION

Dye-determined amniotic fluid volume accurately reflects actual amniotic fluid volume but the dye-determined concentrations, in vivo, may undergo rapid changes.

Determination of amniotic fluid volume in twin pregnancies: ultrasonographic evaluation versus operator estimation

OBJECTIVE

We sought to determine the accuracy of amniotic fluid volume estimation (visually) in diamniotic twin pregnancy versus ultrasonography techniques.

STUDY DESIGN

In this prospective study the volume of each sac in 23 sets of diamniotic twin pregnancies was subjectively and objectively estimated by a second-year obstetric resident, nurse sonographer, maternal-fetal medicine fellow, and maternal-fetal medicine staff. The actual volume was confirmed by amniocentesis and a dye-dilution technique.

RESULTS

There was no difference in the total number of correct estimates of volume by level of operator experience (P =.98), ultrasonography technique (P =.87), or combined subjective versus objective correct estimates (P =.87). Identification of low volume was not different among the four evaluators (P =.48), but the percentage of correct estimates was poor (7%-29%). The 2-diameter pocket was a better predictor of oligohydramnios (57%) than the amniotic fluid index or the largest vertical pocket (12.5%; P =.002).

CONCLUSION

The extremes of volume (low or high) are poorly identified by the subjective or objective assessment of volume.

The amniotic fluid index in normal twin pregnancies

OBJECTIVE

We sought to investigate the amniotic fluid index for individual gestational sacs of twin pregnancies.

STUDY DESIGN

Four hundred eighty-eight patients with normal diamniotic twins were examined between 14 and 40 weeks' gestation. The dividing membrane between twin fetuses was identified. An amniotic fluid index was then obtained for each gestational sac.

RESULTS

The median amniotic fluid index in individual twin gestational sacs rises slowly from 14 to 16 weeks' gestation to 23 to 28 weeks' gestation and then gradually declines. The median amniotic fluid index values by gestational age for twin A and twin B are not statistically different. Although twin pregnancies have a slightly lower median amniotic fluid index value than singleton pregnancies, the difference is also not statistically significant.

CONCLUSION

Individual amniotic fluid indices can be obtained in twin pregnancies, and the values are comparable with those of singleton gestations.

Amniotic fluid volume assessment in singleton and twin pregnancies

The best method of ultrasonic mensuration to identify abnormal AFV reliably in singletons and the individual sacs of diamniotic twins remains elusive. With respect to twins, localization of the dividing membrane seems to be necessary for AFV assessments of each amniotic cavity. The relationship of ultrasonic estimates, actual AFV, and pregnancy outcome remains undetermined. In the authors' opinion, the subjective assessment in twin gestation may be as accurate as semiquantitative ultrasonic estimates of AFV, similar to the situation in singleton pregnancies. The authors propose that further prospective research address the following questions: (1) What is the appropriate threshold for intervention when AFI decreases to lower ranges? (2) What other factors (e.g., cervical examination, fetal heart rate patterns, underlying obstetric conditions, fetal growth pattern) are important when the AFI falls to low values in making clinical decisions?