Clinical vs. sonographic estimate of birth weight in term parturients - A randomized clinical trial

OBJECTIVE: To determine the relative accuracy of clinical and sonographic b irth weight estimation among term parturients (greater than or equal to 37 weeks) and to assess the performance of the two techniques in identifying n ewborns with weights of < 2,500 g vs. greater than or equal to 2,500 g or < 2,500 g vs. at least 4,000 g. STUDY DESIGN: The sample size for this randomized clinical trial was based on the assumption that 50% of clinical predictions are within 10% of birth weight. Thus, 700 parturients were necessary to show a difference of 10% wi th sonographic estimates (alpha=.05, beta=.02). Inclusion criteria were sin gletons with a reliable gestational age of greater than or equal to 37 week s, admitted for delivery and with no known fetal anomalies. Physicians who were unaware of previous sonographic estimates obtained the estimates. Stud ent t and chi 2 tests were used; relative risk (XX) and 95% confidence inte rvals (CIs) were calculated. Receiver-operating characteristic (ROC) curves were constructed to compare the two techniques' ability to differentiate b etween abnormal (birth weight < 2,500 g and 4,000 g) and normal (2,500-3,99 9 g). P <.05 was considered significant. RESULTS: Over 30 months, 758 term parturients were recruited; of them, 391 had clinical estimates and 367, sonographic. The two groups were similar in gestational age, prepregnancy and intrapartum body mass index, station of the presenting part, actual birth weight and frequency of newborns with wei ghts < 2,500 g or greater than or equal to 4,000 g. Predictions based on cl inical examination were significantly more likely to be within 10% of actua l weight (58%) than those derived from ultrasound examination (32%; P<.0001 ; RR, 1.65; 95% CI, 1.43, 1.69). The areas under the ROC curves indicated t hat both techniques had a similar ability to differentiate normally and abn ormally grown fetuses (P >.05). CONCLUSION: Among term parturients, clinical estimates had significantly hi gher accuracy than ones derived sonographically.