Ultrasound assessment of amniotic fluid

Ultrasound monitoring of amniotic fluid volume was first described in 1980 when a subjective classification of 'normal', 'reduced' or 'absent' amniotic fluid was described, based on the presence or absence of echo-free space between the fetal limbs and the fetal trunk or the uterine wall [34]. To test the value of the classification, 150 patients with pregnancies of 42 weeks or more duration underwent ultrasound examination in the 48 h prior to delivery. The patients classified as having 'reduced' or 'absent' amniotic fluid had a statistically significant excess incidence of meconium stained liquor, fetal acidosis, and birth asphyxia and meconium aspiration. Manning et al. [35] described a semiquantitative method based on the largest vertical pool of amniotic fluid and used a 1-cm pool depth as the cut-off for intervention in a population of babies with suspected growth retardation. This was subsequently modified to 2 cm to improve detection of the growth retarded infant [36]. Crowley et al. [37] found an increase in adverse outcomes in post-term pregnancies where the maximum pool depth was less than 3 cm. Fischer et al. [38] found that a maximum vertical pool of less than 2.7 cm was the best predictor of abnormal perinatal outcome.

Phelan [39] described the 'amniotic fluid index', which is the sum of the maximum pool depth in four quadrants. Fischer et al. [38] found that maximum pool depth performed better than amniotic index in predicting adverse outcomes in post-term pregnancies. Alfirevic [40] randomly allocated women with post-term pregnancy to monitoring using either maximum pool depth or amniotic fluid index (AFI). Both groups underwent computerized fetal heart rate monitoring every 3 days in addition to amniotic fluid measurements. The threshold for intervention was a maximum pool depth of less than 1.8 cm or an AFI of less than 7.3 cm. These figures had been identified as the 3rd centiles for the local population. The number of women found to have an abnormal AFI was significantly higher than the number found to have an abnormal maximum pool depth and more women underwent induction of labour in the AFI arm of the trial. There were no perinatal deaths and no statistically significant differences in perinatal outcome between the two groups.

Morris et al. [41] performed an observational study of 1584 pregnant women at or beyond 40 weeks' gestation in Oxford. Women underwent measurement of amniotic fluid, using both the single deepest pool and AFI. The results of these ultrasound measurements were concealed from caregivers. These authors agreed with Alfirevic et al. [40] that more women 'test positive' using AFI with single deepest pool. One hundred and twenty-five women (7.9%) had an AFI of less than 5 cm in contrast to 22 women (1.4%)

who had single deepest pool less than 2 cm. There were no perinatal deaths. There were seven cases of severe perinatal morbidity - an incidence of 0.44%. Two of these had an AFI of <5 cm and four had an AFI of less than 6 cm. None of the seven cases had a deepest pool measurement of less than 2 cm.

Locatelli et al. [42] conducted a similar study, but measured AFI twice weekly from 40 weeks until delivery. A composite adverse outcome of fetal death, 5 min Apgar <7, umbilical artery pH of <7, Caesarean section for fetal distress, occurred in 19.8% of those with an AFI of <5 compared with 10.7% of those with an AFI of >5 (P = 0.001).

These studies of amniotic fluid after 40 weeks suggest some association between reduction in volume and adverse outcome, but overall it performs with a poor sensitivity and specificity. There is no evidence to suggest that it can be relied on as a means of monitoring pregnancies after 41 weeks' gestation. In a meta-analysis of studies on the relationship of amniotic fluid with adverse fetal outcome, Chauhan et al. [43] concluded that there was some association between oligohydramnios and an increased risk of Caesarean section for non-reassuring fetal heart rate patterns and low Apgar scores; however, the data relating to neonatal acidosis were insufficient. The decline in confidence in ultrasound assessment of amniotic fluid volume is compounded by studies which show a poor correlation between ultrasound AFI and actual amniotic fluid volumes measured by dye dilution studies [44,45].

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