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Value of the Amniotic Fluid Index Compared With the Single Deepest Pocket

Department of Obstetrics and Gynecology, University of Mississippi Medical Center, Jackson, Mississippi

In Reply:

We would like to thank Dr. Michael Ross, a well-known amniotic fluid volume researcher, for his letter to the editor about our recent article comparing the measurements of the amniotic fluid volume as a component of the biophysical profile.¹ We agree with Dr. Ross that the importance of the ultrasound assessment of the amniotic fluid volume is not to predict which patient will be intolerant of labor but to prevent adverse clinical outcomes (stillbirth) should the patient not be induced. But, to our knowledge, there are no publications that have concluded the use of amniotic fluid index (AFI) over single deepest pocket decreases the rate of asphyxia or stillbirth. Unfortunately, because no one observes patients whose ultrasound estimate of amniotic fluid volume is low, it is not possible to determine if a stillbirth would have occurred in a trial judging one ultrasound-estimated amniotic fluid volume against another volume.

The utilization of the single deepest pocket as a component of the biophysical profile has a long record of predictability of an adverse antepartum outcome. The false negative fetal death rate using the biophysical profile (BPP) was calculated in 19,221 referred high-risk pregnancies to be 0.726/1,000,² and that rate was subsequently confirmed in 86,955 fetuses in an 18-year experience.³ Additionally the single deepest pocket, as a stand-alone test, has been assessed and the measurements’ predictability of perinatal mortality has been documented (single deepest pocket > 2 cm: 1.91/1,000; single deepest pocket 1–2 cm: 37/1,000; and single deepest pocket < 1 cm: 109/1,000).⁴ The AFI has never been used as a stand-alone test.

The recent study by Morris et al⁵ is an important contribution to our knowledge about amniotic fluid volume estimation in a postterm pregnancy. The authors concluded that the AFI was superior to the single deepest pocket, but the shortcomings of this study must be noted. The authors failed to define the primary outcome of birth asphyxia and meconium aspiration and combined these 2 outcomes as though they had a similar prognosis. Their reported likelihood ratio of 3.3 (2–5 is slightly useful) was only applicable in the prediction of an umbilical artery pH less than 7.00 and was based on the 58% of infants with a cord pH available for evaluation. Meta-analysis on AFI noted that about 23,000 parturients are necessary to demonstrate that AFI of 5.0 cm or less is associated with 1.5 times higher risk of pH less than 7.00 than in patients with adequate AFI.⁶ Unless we have a sufficiently large study, it is futile to think that AFI is a reliable predictor of asphyxia. It was apparent, in this study, that the ultrasound estimation of amniotic fluid volume needs to be combined with other markers of fetal well-being and should not be used as a stand-alone test.

Our investigation confirmed that the AFI was not superior to the single deepest pocket in detecting adverse outcome but labeled twice as many pregnancies as high risk, thus increasing interventions without improving outcome. We believe that the long-documented history of the use of the single deepest pocket with the other components of the BPP have validated its use in the antepartum surveillance of a high-risk pregnancy. Our study confirms the continued use of this measurement, rather than the AFI, in predicting adverse intrapartum and neonatal outcomes.

doi:10.1097/01.AOG.0000149844.08441.a7

REFERENCES

1. Magann EF, Doherty DA, Field K, Chauhan SP, Muffley PE, Morrison JC. Biophysical profile with amniotic fluid volume assessments. Obstet Gynecol 2004;104:5–10.[Abstract/Free Full Text]

2. Manning FA, Morrison I, Harman CR, Lange IR, Menticoglou S. Fetal assessment based on fetal biophysical profile scoring: experience in 19,221 referred high-risk pregnancies. II. An analysis of false-negative fetal deaths. Am J Obstet Gynecol 1987;157:880–4.[Medline]

3. Dayal AK, Manning FA, Berck DJ, Mussalli GM, Aliva C, Harman CR, et al. Fetal death after normal biophysical score: an eighteen-year experience. Am J Obstet Gynecol 1999;181:1231–6.[Medline]

4. Chamberlain PF, Manning FA, Morrison I, Harman CR, Lange IR. Ultrasound evaluation of amniotic fluid volume. I. The relationship of marginal and decreased amniotic fluid volumes to perinatal outcome. Am J Obstet Gynecol 1984;150:245–9.[Medline]

5. Morris JM, Thompson K, Smithey J, Gaffney G, Cooke I, Chamberlain P, et al. The usefulness of ultrasound assessment of amniotic fluid in predicting adverse outcome in prolonged pregnancy: a prospective blinded observational study. BJOG 2003;110:989–94.[Medline]

6. Chauhan SP, Sanderson M, Hendrix NW, Magann EF, Devoe LD. Perinatal outcome and amniotic fluid index in the antepartum and intrapartum periods: a meta-analysis. Am J Obstet Gynecol 1999;181:1473–8.[Medline]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Magann, E. F. Articles by Morrison, J. C. Search for Related Content PubMed Articles by Magann, E. F. Articles by Morrison, J. C.