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Notes: Objectives To establish the relation between fetal abdominal circumference and birthweight in a large population of fetuses; to identify whether the error in estimating birthweight by abdominal circumference varied with the magnitude of abdominal circumference; and to establish whether adding femur length to abdominal circumference caused a clinically important reduction of error in predicting birthweight.Design A retrospective study.Setting The ultrasound department of a teaching maternity hospital offering a tertiary referral service.Sample From 3512 nondiabetic women with a normally formed singleton fetus, an abdominal circumference measurement of the infant was made within seven days of delivery; of these, 1213 had a femur length measurement performed at the same time.Results There was a linear relation between abdominal circumference and birthweight. There was a strong inverse correlation between the proportional error in predicting birthweight from the abdominal circumference and the magnitude of the abdominal circumference. Both the Campbell and Wilkin equation (abdominal circumference alone) and the Hadlock equation (abdominal circumference and femur length) were associated with systematic errors, especially with larger birthweight infants. The median absolute errors for the two equations were not significantly different overall (6.98% and 6.86% respectively), although the Hadlock equation was significantly more accurate in predicting birthweight in infants weighing greater than 4500 g. However, no threshold value of abdominal circumference or of estimated fetal weight using the Hadlock equation had a positive predictive value in estimating infants of 〉 4500 g of greater than 35%.Conclusions Prediction of birthweight should be by abdominal circumference alone. 〈link href="#t1"〉Table 1 presents robust estimates of the error of predicting birthweight using fetal abdominal circumference.〈tabular xml:id="t1"〉1〈title type="main"〉 The relation between fetal abdominal circumference (AC) and birthweight (BW). 〈table colsep="0" rowsep="0" frame="topbot" pgwide="0" orient="port"〉〈tgroup cols="5" align="left"〉〈colspec colnum="1" colname="col1" align="left"/〉〈colspec colnum="2" colname="col2" align="center"/〉〈colspec colnum="3" colname="col3" align="center"/〉〈colspec colnum="4" colname="col4" align="center"/〉〈colspec colnum="5" colname="col5" align="center"/〉〈thead valign="bottom"〉〈row rowsep="1"〉AC (mm)nMedian BW(g)10th-90th centile BW (g)Range BW(g)〈tbody valign="top"〉200-20913900750-1030740-1040210-219201040830-1370780-1400220–229201060750-1410650-1460230-239281255980-1470900-1860240-2493614351200-17901080-1950250-2593715801290-19251180-2260260-2695618351490-21901340-2400270-2798920001640-23201390-2620280-28913422651920-26601530-2910290-29921925302130-29001820-3100300-30935026852340-30802010-3420310-31938728502470-32902110-3650320-32948430602700-34702350-3770330-33943932602880-37002570-3980340-34942333803040-38602670-4240350-35931436153240-40402890-4460360-36924537503330-11903020-4610370-37911738403480-43603180-4790380-3896641403660-46403470-4820390-3993542903665-46753640-5000