Journal of the New Zealand Medical Association, 08-September-2006, Vol 119 No 1241
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Reliability of ultrasound estimation of fetal weight in term singleton pregnancies
Atalie Colman, Dushyant Maharaj, John Hutton, Jeremy Tuohy
To assess the reliability of ultrasound estimation of fetal weight undertaken antenatally at Wellington Hospital (Wellington City, New Zealand) in women with a singleton pregnancy greater than and equal to 37 weeks gestation.
Data were collected retrospectively for pregnant women who had undergone ultrasound estimation of fetal weight <7 days prior to a term delivery (37 weeks gestation) over the period of July 1998–June 2005. Stillbirths and multiple pregnancies were excluded. Ultrasound fetal weight estimations, calculated using a locally modified Woo formula, were compared with the infant’s actual birth weight.
A total of 1177 infants were studied. The mean absolute error and mean signed error (plus or minus SD) of ultrasound fetal weight estimations were 7.0 plus or minus 5.7% and
-0.2 plus or minus 9.0%, respectively (n=1177). Three-quarters of estimations were within 10% of birth weight. Ultrasonic estimation of fetal weight tended to overestimate the weight of small infants (<2500 g; mean signed error = +3.5% plus or minus 9.1%, n=98) and underestimate the weight of large infants (greater than and equal to 4000 g; mean signed error = -3.3 plus or minus 8.7%, n=170). Both large and normal weight infants of women with diabetes tended to have their weight underestimated (mean signed error = -5.1 plus or minus 9.2%, n=48).
Sensitivity, specificity, positive predictive value, and negative predictive value for ultrasonic detection of fetal weight greater than and equal to 4000 g in non-diabetic women were 61%, 96%, 69%, and 94%, respectively. For detection of fetal weight greater than and equal to 4500, the figures were 50%, 98%, 47%, and 98%, respectively.
The accuracy of ultrasound estimations of fetal weight performed at Wellington Hospital within 7 days of delivery in term singleton pregnancies was at least similar and sometimes better than that reported in other studies. For one in four women, however, the fetal weight estimation was more than 10% different from the actual birth weight of their infant. Ultrasound measurements had a tendency to overestimate the weight of small infants while underestimating the weight of both large infants and the infants of diabetic mothers. As the reliability of ultrasound estimation of fetal weight to detect larger babies was poor, the use of such an objective measurement in the management of suspected macrosomia in term singleton pregnancies should be avoided.
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