Estimating fetal weight

Journal of Family Practice,  May, 1998  by Ethan M. Berke,  Lynn M. Oliver

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Sherman DJ, Arieli S, Tovbin J, Siegel G, Caspi E, Bukovsky I. A comparison of clinical and ultrasonic estimation of fetal weight. Obstet Gynecol 1998; 91:212-7.

Clinical question Is routine antepartum ultrasound more accurate than clinical examination for estimating fetal weight?

Background Fetal weight can be assessed clinically by external palpation of the uterus and fetal parts or sonographically using fetal measurements. Of clinical fetal weight estimates, 69% are within 10% of the actual birthweight (BW). Many clinicians contend that ultrasonic examination is more objective and thus a more accurate way to obtain an estimated fetal weight (EFW). Previous studies attempting to compare clinical examination with ultrasonography for determining EFW have had mixed results.

Population studied Women admitted for delivery to an obstetrical unit with gestational ages between 24 and 43 weeks were enrolled. Inclusion criteria included singleton pregnancy, intact membranes, and a routine ultrasound for EFW within 1 week before admission to the delivery suite.

Study design and validity This was a prospective, blinded, noncontrolled study comparing the accuracy of clinical examination with obstetrical ultrasound in determining EFW. Clinical EFW was done by a senior resident without access to the prenatal record or ultrasound results. There was no standardized method to the clinical examination. Actual BW was obtained immediately postpartum. The percentage error, absolute percent error, and ratio of estimates within 10% of the actual BW were calculated for both ultrasound and clinical examination.

Because patients were examined at admission for delivery (85% were at term), the examiner had some knowledge of gestational age that may have influenced the clinical EFW. The authors acknowledge that a relatively accurate EFW can be obtained without even examining the patient if the gestational age is known. Ultrasounds were obtained in several locations and the formulas used to calculate EFW varied.

Outcomes measured The primary outcome measured was EFW stratified to three BW categories: [is less than] 2500 g, 2500 to 4000 g, and [is greater than] 4000 g. The percentage error and percent of estimates of EFW within 10% of the actual BW were reported and compared for each of the three groups.

Results A total of 1717 women received routine ultrasound for EFW within 1 week before admission and clinical EFW. In the [is less than] 2500-g BW group, both methods overestimated actual BW (percentage error by ultrasound 6.8%, clinical 10.0%, P [is less than] .05). The proportion of estimates within 10% of the actual BW was greater with ultrasound (ultrasound 63%, clinical 49%, P [is less than] .05). In the 2500- to 4000-g group, clinical estimation resulted in a statistically significant higher percentage of estimates within 10% of the actual BW (ultrasound 70.6%, clinical 75.1%, P [is less than] .001). In the [is greater than] 4000-g BW group, both methods underestimated the actual BW (percentage error by ultrasound -8.3%, clinical -8.2%). The proportion of estimates within 10% of the actual BW was similar in both groups (ultrasound 58.8%, clinical 61.3%, no significant difference). When all BWs were grouped together, the proportion of estimates within 10% of the actual BW for ultrasound and clinical examination was 68.7% and 71.5%, respectively (not statistically significant).

Recommendations for clinical practice Clinical estimation of fetal weight is neither significantly more nor less accurate than ultrasound at weights [is greater than] 2500 g. There was a statistically significant difference favoring ultrasound in estimation in weights [is less than] 2500 g. The authors conclude that clinical estimation of fetal weight is sufficient in the term or near-term pregnancy, whereas ultrasound may be more appropriate for preterm gestations. Knowledge of gestational age in the term patient may account for most of the 70% accuracy rate obtained by physical examination.

The clinical question of more importance is whether determining EFW is helpful in improving maternal or neonatal outcomes. For example, some may argue that an accurate assessment of fetal weight may be useful in managing patients at risk for macrosomia. However, neither estimate of fetal weight was accurate enough to be helpful in the [is greater than] 4000-g group since approximately 40% of estimates were off by more than 10% of actual BW. Further study is warranted to determine if EFW assessment near delivery improves outcomes.

Ethan M. Berke, MD
Lynee M. Oliver, MD
The University of Washington
Seattle

E-mail:eberke@washington.edu

COPYRIGHT 1998 Dowden Health Media, Inc.
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