Amniotic fluid index compared with single deepest vertical pocket measurement in predicting an adverse pregnancy outcome

Amniotic fluid provides a supportive and protective environment for fetal development during pregnancy. A decreased amniotic fluid volume (oligohydramnios) can occur because of fetal anomalies, intrauterine growth restriction, pre-eclampsia or prolonged (post-term) pregnancy. Many caregivers practice planned delivery by induction of labor or caesarean section after diagnosis of decreased amniotic fluid volume at term, to prevent an adverse pregnancy outcome. Ultrasonography is non-invasive and is used widely for the follow up of pregnancy. It can be used to determine amniotic fluid volume by measuring either the amniotic fluid index or single deepest vertical pocket. 

This review demonstrated that using the amniotic fluid index increased the number of pregnant women who were diagnosed with oligohydramnios and induced for an abnormal fluid volume when compared with the deepest vertical pocket measure. The women also had a higher rate of caesarean section for so-called fetal distress. Yet the rate of admission to neonatal intensive care units and the occurrence of neonatal acidosis, an objective assessment of fetal well-being, were similar between the two groups. The other measured perinatal outcomes that were no different were a non-reassuring fetal heart rate tracing, the presence of meconium, or an Apgar score of less than 7 at five minutes. These conclusions were from five randomized controlled trials involving 3226 women with singleton pregnancies, reported on between 1997 and 2004. 

The accurate assessment of amniotic fluid volume by ultrasonography can be influenced by an inexperienced operator, fetal position, the probability of a transient change, and the different ultrasound diagnostic criteria of an abnormal volume.

Authors' conclusions: 

The single deepest vertical pocket measurement in the assessment of amniotic fluid volume during fetal surveillance seems a better choice since the use of the amniotic fluid index increases the rate of diagnosis of oligohydramnios and the rate of induction of labor without improvement in peripartum outcomes. A systematic review of the diagnostic accuracy of both methods in detecting decreased amniotic fluid volume is required.

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Background: 

Amniotic fluid volume is an important parameter in the assessment of fetal well-being. Oligohydramnios occurs in many high-risk conditions and is associated with poor perinatal outcomes. Many caregivers practice planned delivery by induction of labor or caesarean section after diagnosis of decreased amniotic fluid volume at term. There is no clear consensus on the best method to assess amniotic fluid adequacy.

Objectives: 

To compare the use of the amniotic fluid index with the single deepest vertical pocket measurement as a screening tool for decreased amniotic fluid volume in preventing adverse pregnancy outcome.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (January 2009), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2008, Issue 4), MEDLINE (1966 to January 2008) and the metaRegister of Controlled Trials (December 2008). We handsearched the citation lists of relevant publications, review articles, and included studies.

Selection criteria: 

Randomised controlled trials involving women with a singleton pregnancy, whether at low or high risk, undergoing ultrasound measurement of amniotic fluid volume as part of antepartum assessment of fetal well-being that compared the amniotic fluid index and the single deepest vertical pocket measurement.

Data collection and analysis: 

Both authors independently assessed eligibility and quality, and extracted the data.

Main results: 

Five trials (3226 women) met the inclusion criteria. There is no evidence that one method is superior to the other in the prevention of poor peripartum outcomes, including: admission to a neonatal intensive care unit (Risk Ratio (RR) 1.04; 95% Confidence Intervals (CI) 0.85 to 1.26); an umbilical artery pH of less than 7.1; the presence of meconium; an Apgar score of less than 7 at five minutes; or caesarean delivery. When the amniotic fluid index was used, significantly more cases of oligohydramnios were diagnosed (RR 2.39, 95% CI 1.73 to 3.28), and more women had inductions of labor (RR 1.92; 95% CI 1.50 to 2.46) and caesarean delivery for fetal distress (RR 1.46; 95% CI 1.08 to 1.96)

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