The rest of this reference is: 1999 December181(6): pp1473-1478; Amer J Of Obs & Gyn.
Perinatal outcome and amniotic fluid index in the antepartum and intrapartum periods: A meta-analysis [General Obstetrics And Gynecology: Obstetrics] Chauhan, Suneet P. MDa; Sanderson, Maureen PhDb; Hendrix, Nancy W. MDa; Magann, Everett F. MDc; Devoe, Lawrence D. MDd Spartanburg and Columbia, South Carolina, Jackson, Mississippi, and Augusta, Georgia From the Spartanburg Regional Medical Center,a the Department of Epidemiology and Biostatistics, University of South Carolina,b the Department of Obstetrics and Gynecology, University of Mississippi,c and the Department of Obstetrics and Gynecology, Medical College of Georgia.d Received for publication October 2, 1998; revised January 19, 1999; accepted July 29, 1999. Reprint requests: Suneet P. Chauhan, MD, Division of Maternal-Fetal Medicine, Regional Women's Health Care, 853 North Church St, Suite 403, Spartanburg, SC 29303. Abstract OBJECTIVE: Our purpose was to perform a meta-analysis of studies on the risks of cesarean delivery for fetal distress, 5-minute Apgar score <7, and umbilical arterial pH <7.00 in patients with antepartum or intrapartum amniotic fluid index >5.0 or <5.0 cm. STUDY DESIGN: Using a MEDLINE search, we reviewed all studies published between 1987 and 1997 that correlated antepartum or intrapartum amniotic fluid index with adverse peripartum outcomes. The inclusion criteria were studies in English that associated at least one of the selected adverse outcomes with an amniotic fluid index of <=5.0 cm versus >5.0 cm. Contingency tables were constructed for each study, and relative risks and standard errors of their logs were calculated. Fixed-effects pooled relative risks were calculated for groups of studies that were homogeneous, whereas random-effects pooled relative risks were calculated for significantly heterogeneous groups of studies. RESULTS: Eighteen reports describing 10,551 patients met our inclusion criteria. An antepartum amniotic fluid index of <=5.0 cm, in comparison with >5.0 cm, is associated with an increased risk of cesarean delivery for >fetal distress (pooled relative risk, 2.2; 95% confidence interval, 1.5-3.4) and an Apgar score of <7 at 5 minutes (pooled relative risk, 5.2; 95% confidence interval, 2.4-11.3). An intrapartum amniotic fluid index of <=5.0 cm is also associated with an increased risk of cesarean delivery for fetal distress (pooled relative risk, 1.7; 95% confidence interval, 1.1-2.6) and an Apgar score <7 at 5 minutes (pooled relative risk, 1.8; 95% confidence interval, 1.2-2.7). A poor correlation between the amniotic fluid index and neonatal acidosis was noted in the only study that examined this end point. More than 23,000 patients are necessary to demonstrate that the incidence of umbilical arterial pH <7.00 is 1.5 times higher among those with oligohydramnios in labor than among those with adequate amniotic fluid index ([alpha] = 0.05; [beta] = 0.2) CONCLUSIONS: An antepartum or intrapartum amniotic fluid index of <=5.0 cm is associated with a significantly increased risk of cesarean delivery for fetal distress and a low Apgar score at 5 minutes. There are few reports linking amniotic fluid index and neonatal acidosis, the only objective assessment of fetal well-being. A multicenter study with sufficient power should be undertaken to demonstrate that a low amniotic fluid index is associated with an umbilical arterial pH <7.00. ---------------------------------------------------------------------------- ---- Ultrasonographic assessment of amniotic fluid is used frequently to identify fetuses at risk of having adverse outcomes as suggested by the finding of abnormal fluid volumes. Hydramnios is associated with anomalies or aneuploidy,1 whereas oligohydramnios is linked with pulmonary hypoplasia, postural deformity, fetal distress, and perinatal morbidity and death.2 In 1987 Phelan et al 3 described the amniotic fluid index (AFI) as the summation of the largest vertical pocket in 4 quadrants. This technique of assessing amniotic fluid volume has become increasingly popular in obstetric practice. A MEDLINE search for reports published from 1987 to 1997 includes 125 publications with AFI as the subject. A recent Technical Bulletin on obstetric ultrasonography from The American College of Obstetricians and Gynecologists 4 states that AFI is a more accurate and reproducible method of determining abnormalities in amniotic fluid volume than are other techniques. An AFI <=5.0 cm, consistent with most ultrasonographic criteria for oligohydramnios, has been used as an indication for delivery of infants at or near term. This practice has been suggested by Rutherford et al 5 and by Sarno et al,6 who noted a significantly higher risk of cesarean delivery for fetal distress and low Apgar scores for those parturients with an AFI <=5.0 cm than for those with an AFI >5.0 cm. Since these initial publications, other investigators have not consistently confirmed the association of adverse peripartum outcomes with an AFI <=5.0 cm.7-9 The purpose of this meta-analysis was to determine the relative risks of cesarean delivery for fetal distress, 5-minute Apgar score <7, and umbilical arterial pH <7.00 among patients with AFI >5.0 or <=5.0 cm. Material and methods A MEDLINE search was conducted for all English-language publications involving human subjects with the key words or phrases being amniotic fluid index, oligohydramnios, and amniotic fluid. The search was limited to the years 1987-1997, since the index publication by Phelan et al,3 which introduced the concept of AFI in 1987. Other inclusion criteria for the studies considered in the meta-analysis were (1) publication in a peer-reviewed journal, (2) determination of AFI by the technique of Phelan et al,3 (3) criteria for oligohydramnios as AFI <=5.0 cm, and (4) reporting of the incidences of cesarean delivery for fetal distress, or Apgar score <7 at 5 minutes, or umbilical arterial pH <7.00 at the AFI thresholds specified earlier. If a study that combined operative vaginal and cesarean deliveries for fetal distress in 1 group reported the incidence of low Apgar scores or of neonatal acidosis, then the study was included for the analysis of the latter 2 outcomes. If there was >1 publication with the same investigators and institution, then the one with the largest sample size was included in the analysis. Two of the authors (S.P.C. and N.W.H.) reviewed each publication independently to determine its compliance with inclusion criteria. All studies that met the inclusion criteria were next grouped by the timing of AFI assessment, as obtained during either the antepartum or the intrapartum period. The meta-analyses of these 2 groups were done separately. >From 1987 to 1997 there were 42 articles that correlated AFI and peripartum outcome. Thirteen reports were excluded from analysis because the investigators did not define oligohydramnios with an AFI <=5.0 cm. Six reports were excluded because they did not identify individually the 3 adverse outcomes of interest. Four studies were excluded because the same authors had publications from the same institution with a larger sample size. One report (reference available from the primary author, S.P.C.) was not incorporated in the meta-analysis because the authors did not adhere to the technique of Phelan et al.3 This resulted in the inclusion of 18 studies 5-22 that met all inclusion criteria. After exclusion of the only case-control study,18 incidences and 95% confidence intervals were calculated for abnormal conditions and for each of the subgroups related to 1 of the 4 adverse outcomes. On the basis of the numbers of patients with AFI <=5.0 cm and the particular outcome of interest, a 2 × 2 table was constructed for each study. We added 0.5 to 0 cells, and the relative risk and standard error of the logs were calculated for each table. For the pooled relative risk, each study was weighted by means of the inverse of the variance of the relative risk. Fixed-effects pooled relative risks were calculated for groups of studies that were homogeneous, whereas random-effects pooled relative risks were calculated for significantly heterogeneous groups of studies. Results Eight publications 5, 9,15-17, 20-22 examined the relationship of antepartum AFI to peripartum outcome. Four of these reports 5, 15, 20, 22 involved high-risk patients for whom antepartum surveillance was indicated; 3 reports focused on patients with a single complication (diabetic pregnancy,17 severe preeclampsia at <34 weeks' gestation,9 or gestational age of >=41 weeks 21); and 1 study 16 included patients between 37 and 42 weeks' gestation with intact membranes but did not specify the indication for assessment of amniotic fluid. All these reports excluded patients with ruptured membranes. These 8 studies 5, 9, 15-17, 20-22 contained 5962 patients with an overall incidence of oligohydramnios of 15.2% (907/5962; 95% confidence interval 14.3-16.1). The prevalence of AFI <=5.0 cm varied from 2.9% (9/311)16 to 41.3% (98/237) (reference available from S.P.C.). Seven of these reports 5, 9, 15-17, 20, 21 identified the risk of cesarean delivery for fetal distress among those with AFI <=5.0 cm; 1 study was excluded because it combined all operative deliveries (vaginal and abdominal) for distress as abnormal outcome.22 In these 7 studies,5, 9, 15-17, 20, 21 635 of 5786 patients had cesarean delivery for fetal distress. Table I describes the relative risk and 95% confidence interval for each of the 7 studies. The pooled relative risk for cesarean delivery for fetal distress associated with antepartum AFI of <=5.0 cm was 2.22 (95% confidence interval, 1.47-3.37). The random-effects pooled relative risk is presented because there appeared to be significant heterogeneity of studies ([chi]2 = 11.41; 6 degrees of freedom; P = .076). Six of these studies 5, 15, 16, 20-22 reported the association of AFI and Apgar score <7 at 5 minutes (132/4325 patients). The relative risk and 95% confidence interval for these studies is described in Table II. The random-effects pooled relative risk of 5.16 (95% confidence interval, 2.36-11.29) is presented because there is evidence of significant study heterogeneity ([chi]2 = 10.85; 5 degrees of freedom; P = .054). None of these 8 studies 5, 9, 15-17, 20-22 reported the incidence of umbilical arterial pH <7.10 or <7.00 among those patients with oligohydramnios or adequate amniotic fluid. ---------------------------------------------------------------------------- ---- [Help with image viewing] [Email Jumpstart To Image] Table I. Antepartum AFI and risk of cesarean delivery for fetal distress ---------------------------------------------------------------------------- ---- ---------------------------------------------------------------------------- ---- [Help with image viewing] [Email Jumpstart To Image] Table II. Antepartum AFI and risk of Apgar score <7 at 5 minutes ---------------------------------------------------------------------------- ---- Ten studies 6-8, 10-14, 18, 19 correlated intrapartum AFI and peripartum complications; 7 of these studies 6, 8, 11-13, 18, 19 examined assessment of amniotic fluid irrespective of maternal or fetal complications. One study 7 involved only patients at >=36 weeks' gestation with ruptured membranes; 1 study 10 focused on uncomplicated pregnancies at 37 to 42 weeks with early labor; and 1 study 14 was concerned only with patients who had antecedent obstetric or medical complications. All studies 6-8, 10-14, 18, 19 determined AFI during hospitalization and excluded patients who underwent elective cesarean delivery or had fetal distress on admission that required prompt delivery. These 10 studies 6-8, 10-14, 18, 19 contained >4500 parturients with an overall incidence of AFI <=5.0 cm of 21.9% (1004/4589; 95% confidence interval, 20.7-23.1); the range of oligohydramnios was 4.1% (37/893)10 to 37.8% (169/447).12 Of these 10 reports, 9 involved >3500 parturients and provided the risk of cesarean delivery for fetal distress among those with an AFI of <=5 cm versus >5.0 cm. One report 10 was excluded from this analysis because it combined all operative deliveries for fetal distress as a single category. Overall the risk of this adverse outcome was 6.3% (230/3672; 95% confidence interval, 5.5-7.1). Table III provides the relative risk and 95% confidence interval for each of these 9 publications.6-8, 11-14, 18, 19 The pooled relative risk for cesarean delivery because of fetal distress associated with an AFI <=5.0 cm was 1.69, with a 95% confidence interval of 1.12-2.57. The exclusion of the only case-control study 18 did not substantially change the pooled relative risk, but because the studies were heterogeneous, the random-effects relative risk is presented ([chi]2 = 16.05; 8 degrees of freedom; P = .042). ---------------------------------------------------------------------------- ---- [Help with image viewing] [Email Jumpstart To Image] Table III. Intrapartum AFI and risk of cesarean delivery for fetal distress ---------------------------------------------------------------------------- ---- All 10 reports 6-8, 10-14, 18, 19 provided the risk of an Apgar score <7 at 5 minutes among the 2 groups. A total of 4565 parturients delivered 101 newborn infants (2.2%; 95% confidence interval, 1.8-2.7) who were depressed at 5 minutes. Table IV presents the relative risk and 95% confidence interval for each of the studies. We excluded the only case-control study 10 because its inclusion substantially changed the pooled relative risk. After its exclusion, the studies were homogeneous ([chi]2 = 3.5; 9 degrees of freedom; P = .089). ---------------------------------------------------------------------------- ---- [Help with image viewing] [Email Jumpstart To Image] Table IV. Intrapartum AFI and risk of Apgar score <7 at 5 minutes ---------------------------------------------------------------------------- ---- Only 1 study 14 reported the risk of neonatal acidosis and amniotic fluid status in early labor. Among high-risk parturients with an AFI <=5.0 or >5.0 cm, the incidence of umbilical arterial pH <7.10 is not significantly different, 3.3% (6/184) and 7.2% (22/306; P = .07; relative risk, 0.5; 95% confidence interval, 0.2-1.1). There was also no difference in the incidence of umbilical arterial pH <7.00 among those with an AFI <=5.0 cm (0.5%; 1/184) or >5.0 cm (2.6%; 8/306) (P = .16, relative risk, 0.3; 95% confidence interval, 0.4-1.8). Comment In term pregnancies oligohydramnios, defined by an ultrasonographic determination of AFI <=5.0 cm, has been suggested as an indication for delivery.14, 15, 18 Ten years after Phelan et al 3 described the semiquantitative method of determining the AFI, it is reasonable to reassess the evidence to support the criterion of an AFI <=5.0 cm as a clinical cutoff point for intervention. Our meta-analysis showed that the use of the AFI for clinical decision making is problematic. First, the incidence of AFI <=5.0 cm, whether obtained in the antepartum or the intrapartum period, varied widely (3%-40%).17, 22 A number of factors could have accounted for this finding: (1) different patient populations with varying prevalences of complications, (2) interobserver variability and technique of insonation, and (3) diverse obstetric factors that influence the actual volume of amniotic fluid. For example, among pregnant women with diabetes,17 the incidence of oligohydramnios is 3%, but among patients with severe preeclampsia,9 it is 20%. In a community hospital an AFI <=5.0 cm was reported in 12% of cases,11 whereas at tertiary centers the incidence was 38%.12 Gestational age influences the normal distribution of AFI, resulting in significant differences for preterm, term, and postterm pregnancy.23 Undeniably, there is interobserver variability when the deepest vertical pockets are measured in 4 quadrants, particularly at the lower end of amniotic fluid volumes.23 It would be impractical in clinical care to perform multiple repetitions of this assessment to minimize this source of error. Further, no prospective report has found an improvement in the prediction of adverse outcomes when repeated measures of amniotic fluid were used. Last, transducer pressure, the ambient temperature, altitude, diabetic glucose control, and the status of maternal hydration and amniotic membranes may influence AFI (reference available from S.P.C. on request). Considering the potential number of confounding variables, it is not surprising that the reported incidence of low AFI varies widely. The second major finding of this meta-analysis is that, whether the amniotic fluid is assessed in the antepartum or the intrapartum period, an AFI <=5 cm, in comparison with an AFI >5.0 cm, is associated with an increased risk of cesarean delivery for fetal distress and an Apgar score <7 at 5 minutes (Tables I-IV). Are these findings sufficient evidence that an AFI <=5.0 cm reflects a compromised fetus who needs to be delivered if at term? Admittedly, other factors are associated with an increased risk of abdominal delivery or a depressed neonate. For example, the decision to proceed with cesarean delivery because of fetal distress is based on factors such as subjective interpretation of fetal heart rate tracing, the time of day and the day of the week, the underlying medical complication, the gender of the fetus, whether the parturient is managed by midwives or physicians, whether continuous electronic tracing or intermittent auscultation is used during labor, and whether the AFI is known.12, 24 Thus cesarean delivery for fetal distress would be preferable only after a fetal scalp pH value is obtained. However, because of cervical dilatation, non-availability of the machine, or other constraints, the fetal pH may not be attainable before emergency cesarean delivery. Only 2 of the antepartum studies 15, 16 and 1 of the intrapartum studies 7 that met the inclusion criteria mentioned that scalp pH values were used in the investigators' practices. However, none of these 3 reports 7, 15, 16 provided information about the results of scalp pH determinations among those with or without an adequate AFI. A low Apgar score may be the result of use of narcotics in labor, preterm birth, or vigorous suctioning of the neonate. With a meta-analysis, it is not possible to account for all these confounding variables and adverse outcomes. The possibility of treatment paradox with antenatal tests for fetal well-being and peripartum outcome should also be considered.25 According to this concept, the outcome of diagnostic testing can be falsely improved because an abnormal finding (oligohydramnios) leads to a series of interventions (induction) that result in adverse outcomes (cesarean delivery) that the test is supposed to prevent. The only study 12 to randomize knowledge of AFI in laboring patients confirmed that awareness of the status of amniotic fluid is associated with an increased incidence of induction and cesarean section for fetal distress, without the benefit of improved neonatal outcome. The third and most important finding of this analysis is the lack of data on the possible association between AFI and neonatal acidosis, the only objective means to assess fetal well-being. Previously, umbilical arterial pH <7.20 was consistent with acidosis in a newborn infant.26 Six reports 9, 14, 15, 18-20 included in the analysis noted an investigation of AFI and pH <7.20. Three of these studies 9, 15, 20 obtained the AFI in the antepartum period, and the remaining 3 14, 18, 19 during early labor. None of them reported a significantly increased risk of umbilical arterial pH <7.20 with an AFI of <=5 cm. In the largest study correlating antepartum AFI and umbilical arterial pH <7.20, Hoskins et al 15 noted an increased risk of acidosis only if an AFI <=5.0 cm is associated with severe variable decelerations. Recent evidence indicates that injury related to hypoxic-ischemic encephalopathy does not occur unless the umbilical arterial pH is <7.00.26 Only 1 study 14 determined the link between intrapartum AFI and umbilical arterial pH <7.00, and it showed a poor correlation between the 2. If pathologic acidemia occurs in 0.7% of newborn infants, then approximately 11,600 patients are necessary in each group to demonstrate that, in comparison with those having an AFI >5.0, the incidence of umbilical arterial pH <7.00 is 1.5 times higher among those patients with oligohydramnios ([alpha] = 0.05; [beta] = 0.2; power of 80%). Until such a study is performed, smaller reports with periodic cumulative meta-analysis will have to suffice. Besides AFI, other methods of ultrasonographic assessment of AFI include subjective assessment, single deepest pocket, an isolated pocket of at least 2 × 2 cm or 1 × 1 cm, and 2-diameter pocket.27 The only randomized study to compare 2 techniques was by Alfirevic et al.28 These investigators randomly allocated patients at >=42 weeks to have either computer cardiotocography with AFI determination or computer cardiotocography with maximum pool depth. These authors concluded that, although the neonatal outcomes were similar in both groups, the incidence of oligohydramnios and subsequent inductions was significantly higher among those who had AFI determinations rather than deepest vertical pocket measurement. Thus, instead of the AFI, alternative techniques of ultrasonographically assessing amniotic fluid could be used in clinical practice. Our meta-analysis has some frank limitations. Most important was our decision to limit inclusion to studies that defined oligohydramnios as an AFI <=5.0 cm, whereas other reports have set different AFI thresholds as more appropriate (references available from S.P.C.). To keep the analysis consistent, we chose <=5.0 cm as the threshold because that is what the original report suggested 3 and because it is the commonly used definition of the abnormal condition. A separate meta-analysis should evaluate the risk of adverse outcomes at thresholds other than an AFI of <=5.0 cm. It should be acknowledged that the quality of the meta-analysis is based on the nature of the studies that are included. We conclude that an AFI <=5.0 cm is associated with an increased risk of cesarean delivery for fetal distress and low Apgar scores at 5 minutes. This meta-analysis permits the clinician to gauge the relative risk of adverse outcomes with oligohydramnios, using the criteria of Phelan et al,3 and points out the lack of reports linking AFI and neonatal acidosis. A large multicenter, randomized study is warranted to determine the benefits of using AFI in antepartum surveillance or in early labor. REFERENCES 1. Brady K, Polzin WJ, Kopelman JN, Read JA. Risk of chromosomal abnormalities in patients with idiopathic polyhydramnios. Obstet Gynecol 1992;79:234-8. Bibliographic Links [Context Link] 2. Golan A, Lin G, Evron S, Arieli S, Niv D, David MP. Oligohydramnios: maternal complications and fetal outcome in 145 cases. Gynecol Obstet Invest 1994;37:91-5. Bibliographic Links [Context Link] 3. Phelan JP, Smith CV, Broussard P, Small M. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks' gestation. J Reprod Med 1987;32:540-2. Bibliographic Links [Context Link] 4. 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American College of Obstetricians and Gynecologists. Umbilical artery blood acid-base analysis. Washington: The College; 1995. Technical Bulletin No.: 216. [Context Link] 27. Magann EF, Nolan TE, Hess LW, Martin RW, Whitworth NS, Morrison JC. Measurement of amniotic fluid volume: accuracy of ultrasonography techniques. Am J Obstet Gynecol 1992;167:1533-7. Bibliographic Links [Context Link] 28. Alfirevic Z, Luckas M, Walkinshaw SA, McFarlane M, Curran R. A randomized comparison between amniotic fluid index and maximum pool depth in the monitoring of post-term pregnancy. Br J Obstet Gynaecol 1997;104:207-11. [Context Link] Key words: Perinatal outcome; amniotic fluid index; meta-analysis ---------------------------------------------------------------------------- ---- Accession Number: 00000447-199912000-00030 Copyright (c) 2000-2006 Ovid Technologies, Inc. Version: rel10.3.1, SourceID 1.12052.1.95 -- This mailing list is sponsored by ACE Graphics. 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