According to this new ultrasound study we have found out yet again that the birth world is correct and ultrasound
isn’t as accurate as medical practitioners seem to think it is.
Aim 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 ≥37 weeks gestation.
Method 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.
Results A total of 1177 infants were studied. The mean absolute error and mean signed error (±SD) of ultrasound
fetal weight estimations were 7.0±5.7% and -0.2±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%±9.1%, n=98) and underestimate the weight of large infants
(≥4000 g; mean signed error = -3.3±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±9.2%, n=48).
Sensitivity, specificity, positive predictive value, and negative predictive value for ultrasonic detection of fetal
weight ≥4000 g in non-diabetic women were 61%, 96%, 69%, and 94%, respectively. For detection of fetal weight
≥4500, the figures were 50%, 98%, 47%, and 98%, respectively.
Conclusion 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.
To Read the Study Visit