Limitations of clinical and sonographic estimates of birth weight: experience with 1034 parturients.

OBJECTIVE: To compare the accuracy of clinical and sonographic estimates of fetal weight made throughout the third trimester of pregnancy.
METHODS: Patients in early labor had fetal weight estimated by two approaches: 1) clinical evaluation and palpation followed by 2) sonographic mensuration of fetal biparietal diameter, abdominal circumference, and femur length applied to Hadlock's formula. The accuracy of these two methods of estimating fetal weight was compared using Student t test, Wilcoxon test, and chi2 tests. P < .05 was considered significant. Prediction limits (50th, 90th, and 95th percentiles) were calculated for both techniques by obtaining the range of actual weights associated for a particular estimated fetal weight (EFW).
RESULTS: We enrolled 1034 parturients whose clinical EFWs yielded significantly higher mean (+/- standard deviation) simple error (48.2 +/- 411 g) and standardized absolute error (130 +/- 122 g/kg) than were obtained by use of sonographic formulas for EFW (-6.6 +/- 381 g and 104 +/- 89 g/kg, respectively). When the population was partitioned by gestational age, we found that sonographic EFW was more accurate than clinical EFW in preterm (n = 373) but not in term (n = 460) or post-term (n = 201) pregnancies. Prediction limits indicate that for a given EFW, for example, 800 g, the 90% ranges of actual weight based on clinical and sonographic EFW are 566-1829 g and 469-1667 g, respectively.
CONCLUSION: The apparent superiority of sonographic EFW over clinical EFW applies principally to preterm pregnancies. The prediction limitation calculation suggests that either method, for any particular estimate between 500 and 4500 g, has limited value in the estimation of actual birth weight, because this outcome is highly variable and frequently lies outside of the useful bandwidth (+/- 10%) for prospective management.