Normal values for amniotic fluid index during uncomplicated twin pregnancy

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).

Polyhydramnios: Causes, Diagnosis and Therapy

Polyhydramnios is defined as a pathological increase of amniotic fluid volume in pregnancy and is associated with increased perinatal morbidity and mortality. Common causes of polyhydramnios include gestational diabetes, fetal anomalies with disturbed fetal swallowing of amniotic fluid, fetal infections and other, rarer causes. The diagnosis is obtained by ultrasound. The prognosis of polyhydramnios depends on its cause and severity. Typical symptoms of polyhydramnios include maternal dyspnea, preterm labor, premature rupture of membranes (PPROM), abnormal fetal presentation, cord prolapse and postpartum hemorrhage. Due to its common etiology with gestational diabetes, polyhydramnios is often associated with fetal macrosomia. To prevent the above complications, there are two methods of prenatal treatment: amnioreduction and pharmacological treatment with non-steroidal anti-inflammatory drugs (NSAIDs). However, prenatal administration of NSAIDs to reduce amniotic fluid volumes has not been approved in Germany. In addition to conventional management, experimental therapies which would alter fetal diuresis are being considered.

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.

Management of Multiple Pregnancy: Prenatal Care???Part I

Over the past 20 years, the number and rate of multiple births have dramatically increased in the United States. The rise in multiple births is mainly attributable to the increased use of ovulation-inducing drugs and the newly developed assisted reproductive technologies such as in vitro fertilization. Multifetal gestation is associated with an increased risk of perinatal morbidity and mortality. Multiple births account for an increasing percentage of low-birth-weight infants, preterm births, and infant mortality. In this section, we address the management of the multifetal pregnancy, focusing on the maternal physiology, the diagnosis, the pregnancy outcomes, and the antenatal management of multiple gestation.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader should be able to describe the effects of the rising rate of multiple pregnancies on perinatal morbidity and mortality, to recall the complications of diagnosing and treating abnormalities of multiple pregnancies, and to list the many changes that occur in both the mother and the fetuses in multiple pregnancies.

Twin pregnancy: the role of ultrasound in management

Determination of chorionicity is one of the most important issues in the management of twin pregnancy. Modern ultrasound equipment has made it possible to accurately assess placentation already in the first trimester with the lambda sign. With regard to prenatal diagnosis, it is important to know the chorionicity in order to calculate the risk of chromosomally abnormal fetuses. Accurate chorionicity offers the obstetricians the opportunity to observe the monochorionic twins more intensively than is required for twins with dichorionic placentation. This review gives an update of the state of the art for clinicians caring for twin pregnancies.

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.

Antepartum evaluation of monochorionic diamniotic (MD) twins; MD-twin score: a new scoring method for perinatal outcome

OBJECTIVE

Our purpose was to establish a new scoring method to survey monochorionic diamniotic (MD) twins during antepartum periods.

STUDY DESIGN

A retrospective study was performed regarding MD twins delivered between January 1992 and July 1996. Maternal and neonatal records were assessed for the following 5 perinatal variables; birth-weight discordance, amniotic-fluid discordance, hydrops fetalis, umbilical-cord insertion, and fetal-heart-rate monitoring. Each variable was coded as normal or abnormal and then assigned an arbitrary weight of 0 if normal and 1 if abnormal, yielding a range of scores from 0 (all normal) to 5 (all abnormal). The relationships between individual variables and their combinations and the outcome of pregnancy was determined. A poor pregnancy outcome consisted of intrauterine death, neonatal death, or neurological sequelae of at least one twin. The 5-variable combination was termed as the MD-twin score. A chi-square test and logistic regression analysis were used to determine statistical significance.

RESULTS

There were 59 MD pregnancies, of which 13 pregnancies resulted in a poor outcome. The single variable that most likely contributed to a poor outcome was amniotic-fluid discordance. All 35 pregnancies with an MD-twin score of < or = 2 had a good outcome. There were 14 pregnancies with a score of 3, and 21% of them had a poor outcome. All of the pregnancies with a score of > or = 4 had a poor outcome. When we chose the MD-twin score of 3 as the critical point for a poor outcome, the likelihood ratio statistics became the highest of any single variable or any combination of variables.

CONCLUSION

The MD-twin score predicted poor outcomes better than did any single variable or combination of variables.

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.

Placental angioarchitecture in monochorionic twin pregnancies: relationship to fetal growth, fetofetal transfusion syndrome, and pregnancy outcome

OBJECTIVE

We sought to correlate placental vasculature with fetal growth and outcome in monochorionic twins.

STUDY DESIGN

Eighty-two patients with consecutive monochorionic pregnancies underwent biweekly ultrasonography for determination of fetal growth and well-being. After delivery, blinded placental injection studies delineated vascular anastomoses and territory share. Degree of balance in arteriovenous anastomoses equaled the number of arteriovenous anastomoses in one direction minus the number in the other.

RESULTS

Pregnancies affected by fetofetal transfusion syndrome (n = 21) had numbers of arteriovenous and venovenous anastomoses that were similar to those in pregnancies without fetofetal transfusion syndrome but fewer arterioarterial anastomoses (P <.0001). Fetofetal transfusion syndrome occurred in 78% of pregnancies with >/=1 arteriovenous and no arterioarterial anastomoses. Birth weight discordancy correlated with placental territory discordancy (P <.0001) and the degree of balance in arteriovenous anastomoses (P =.004). The larger placental share twin had a greater growth velocity than its smaller placental share co-twin (P =.008) for all but one anastomotic pattern. Where arteriovenous anastomoses were aligned with the net venous outflow to the fetus with the smaller territory, co-twins had similar birth weights and growth velocities irrespective of placental share. Fetal survival was higher in pregnancies with an arterioarterial anastomosis (P =.01) but lower with a venovenous anastomosis (P =. 01). Survival of both fetuses was inversely associated with birth weight discordancy (P <.0001).

CONCLUSION

Although interrelationships among the various types of anastomoses are complex, our data suggest that the placental territory share and the pattern of arteriovenous anastomoses influence fetal growth, that arterioarterial anastomoses protect against fetofetal transfusion syndrome, and that venovenous anastomoses reduce perinatal survival.

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?

Amnioreduction: how much to drain?

OBJECTIVES

To assess the relationship between the volume of amniotic fluid removed and the change in amniotic fluid index (AFI) and calculate an equation describing this association.

MATERIALS AND METHODS

A retrospective analysis of 19 amnioreduction procedures performed in our unit. Multiple regression analysis was used to assess the effect of gestational age and pre-procedure AFI on the change in AFI (DeltaAFI) after adjusting for the volume removed.

RESULTS

As expected, a significant linear relationship was found between the change in AFI and the volume removed (r = 0.82, n = 19, p < 0.0001). DeltaAFI was not dependent on the gestational age or the pre-procedure AFI. The equation describing the association between the volume removed and DeltaAFI was: volume = (DeltaAFI - 2. 26)/0.008, which is close to 1 cm DeltaAFI for every 100 ml removed.

DISCUSSION

Using the described equation, it is possible to predict the required volume to be removed in order to achieve a particular DeltaAFI, which may reduce the need to interrupt the procedure to measure the AFI. However, the limitation of AFI as a semiquantitative assessment of the liquor volume, together with its inter- and intra-observer variations mean this equation should be used only as a guide.

Equalization of amniotic fluid volumes after decompression amniocentesis for treatment of the twin oligohydramnios-polyhydramnios sequence

OBJECTIVE

To measure acute and chronic changes in the placenta and amniotic fluid associated with performance of decompression amniocentesis in pregnancies with the twin oligohydramnios-polyhydramnios sequence (TOPS).

METHODS

Amniotic fluid pressures, placental thickness, placental perfusion, and amniotic fluid volumes were measured in each sac of a monochorionic diamniotic twin gestation before and after decompression amniocentesis. Indigo carmine was injected into the polyhydramnic sac after decompression, and fluid from the oligohydramnic sac was sampled after equilibration. Spectrophotometric analysis of amniotic fluid specimens was performed for dye detection. Amniotic fluid volume and placental perfusion studies were repeated 1 week later.

RESULTS

Three patients with TOPS were enrolled, and decompression amniocentesis was performed in the midtrimester. After decompression, amniotic fluid volume decreased in the polyhydramnic sac, amniotic fluid pressures decreased in both sacs, placental thickness increased, and umbilical artery Doppler velocimetry was unaffected. The amniotic fluid volume increased acutely in only one oligohydramnic sac after decompression, and ultrasonographic examination, amniotic fluid spectrophotometric analysis, and placental pathologic examination all identified interfetal membrane disruption as the etiology.

CONCLUSIONS

Decompression amniocentesis as a treatment for TOPS does not result in acute or chronic changes in the amniotic fluid volume of the oligohydramnic sac in the absence of interfetal membrane disruption.

Sonographic considerations with multiple gestation

Determination of chorionicity is of paramount importance in risk assessment and management. Best performed in the first trimester, dichorionic placentation can be reliably assumed when the membrane is easily seen, there is a "twin peak" sign, there are clearly separate placentas, and there is discordant fetal gender. In a monochorionic twin pregnancy, there is a single placental mass, the dividing membrane is difficult to visualize until the end of the first trimester, and the membrane inserts onto the placental surface without a peaked appearance. Amniotic fluid volume assessment is important in the management of twin pregnancy. Polyhydramnios-oligohydramnios may be a manifestation of twin-twin transfusion syndrome, although oligohydramnios with normal amniotic fluid volume in the other twin's sac may more likely be a sign of velamentous cord insertion, infection, or chromosomal or structural abnormality. Fetal growth discordance is common in twin pregnancy and is associated with increased perinatal mortality and morbidity. The most sensitive indicator of discordant twin growth is thought to be estimated fetal weight, and an intertwin difference of > or = 20% is considered significant. In the clinical care of a patient with twins, it is reasonably standard to confirm chorionicity with ultrasonography in the first or early second trimester. At about 20 weeks, a level II ultrasound for anatomic survey is indicated. In dichorionic pregnancies, ultrasound examinations are then performed at 26 to 28 weeks and every 3 to 4 weeks thereafter to follow growth and amniotic fluid volume. In monochorionic twins, we generally do an additional ultrasound at about 23 to 24 weeks, because of the risk of twin-twin transfusion syndrome. In the late third trimester, careful attention should also be given to fetal position, to help with delivery planning.

Sonographic examination of twins. From first trimester to delivery of second fetus

Sonographic examination is essential in the diagnosis and management of twin gestation. It assists in determining the zygosity, assessing fetal anomalies, and is integral to amniocentesis if it is necessary, determining the growth and ruling out discordance, and in intrapartum management. The management of uncommon complications with twins also requires ultrasonic survey. Considering that the incidence of multiple gestation is increasing, it would be prudent to become familiar with the use and benefit of ultrasound with twins.

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

OBJECTIVE

Our purpose was to determine the accuracy of the summated amniotic fluid index designed to estimate the total amniotic fluid volume in twin pregnancies.

STUDY DESIGN

The summated amniotic fluid index was measured in 62 normal diamniotic twin pregnancies by adding the deepest vertical pockets in the four quadrants. Actual amniotic fluid volume was then determined in all 124 amniotic sacs by amniocentesis and a dye-dilution technique. For data analysis, amniotic fluid volumes were classified by percentile with use of previously reported norms.

RESULTS

There were significant differences in the percentile distribution of amniotic fluid volume as estimated by the summated amniotic fluid index and the actual volume as determined by dye dilution (p < 0.001). The summated amniotic fluid index has a sensitivity of only 13% in predicting amniotic sac volume.

CONCLUSION

The summated amniotic fluid index is a poor predictor of intertwin differences in amniotic fluid volume and cannot identify twin pairs at risk for oligohydramnios and hydramnios.

Effect of fetal movement on the amniotic fluid index in diamniotic twin gestations

Fetal movement changes the size and location of amniotic fluid pockets during measurement of the amniotic fluid index. In singleton gestations, the effect of redistributing the fixed intrauterine fluid volume on the amniotic fluid index is clinically insignificant. In this study, we tested the hypothesis that the index in twin pregnancies is unaffected by fetal movement. A single examiner prospectively determined the amniotic fluid index before and after three discrete episodes of movement by both fetuses of 82 diamniotic twin pregnancies referred for obstetric sonograms between 20 and 38 weeks' menstrual age. A reliable blinded examiner provided a second post-movement measurement as a control. Data were analyzed by the paired t-test. The mean change in the amniotic fluid index after fetal movement was 2.1 +/- 0.2 cm and 3.7 +/- 0.3 cm for post-movement determinations by the same and blinded examiners, respectively (p < .001). Interobserver variation was 3.5 cm. Intraobserver variation was 1.8 cm for the first examiner and 2.2 cm for the second examiner. Therefore, interobserver and intraobserver variation can account for the observed change in the amniotic fluid index following movement of both diamniotic twins.