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Objective: To determine the accuracy of ultrasonographically estimated fetal weight (EFW) and abdominal circumference (AC) in the prediction of macrosomia. Design: Systematic quantitative review. Methods: Studies were identified without language restrictions from MEDLINE (1966-2003), EMBASE (1980-2003), Cochrane Library (2003:4), SCISEARCH (1974-2003) and manual searching of bibliographies of known primary and review articles. Studies were selected if accuracy of ultrasonographically EFW or AC was evaluated for predicting macrosomia using birthweight as the reference standard. Data were extracted on study characteristics, quality and accuracy. Data were pooled to produce summary receiver operating characteristic curves (sROC) for studies with various test thresholds. Summary likelihood ratios for positive (LR+) and negative (LR-) test results were generated for an EFW of 4000 g and an AC of 36 cm for predicting birthweight of over 4000 g. Main Outcome Measures: Birthweight over various thresholds. Results: There were 36 primary articles consisting of 63 accuracy studies (51 evaluating the accuracy of EFW, and 12 accuracy of fetal AC), including a total of 19,117 women. The sROC area for EFW was not different from the area for fetal AC (0.87 vs 0.85, P= 0.91). For predicting a birthweight of over 4000 g, the summary LRs were 5.7 (95% CI: 4.3 to 7.6) for a positive test and 0.48 (95% CI: 0.38 to 0.60) for a negative test, using Hadlock's method of ultrasonographically estimating fetal weight. For ultrasound fetal AC of 36 cm, the respective LRs for predicting a birthweight over 4000 g were 6.9 (95% CI: 5.2 to 9.0) and 0.37 (0.30-0.45). Conclusion: There is no difference in accuracy between ultrasonographically EFW and AC in the prediction of a macrosomic baby at birth. A positive test result is more accurate for ruling in macrosomia than a negative test result for ruling it out. [ABSTRACT FROM AUTHOR]