The relation between fetal abdominal circumference and birthweight: findings in 3512 pregnancies

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Abstract
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. Table 1 presents robust estimates of the error of predicting birthweight using fetal abdominal circumference. The relation between fetal abdominal circumference (AC) and birthweight (BW).: AC (mm) n Median BW(g) 10th‐90th centile BW (g) Range BW(g) 200‐209 13 900 750‐1030 740‐1040 210‐219 20 1040 830‐1370 780‐1400 220–229 20 1060 750‐1410 650‐1460 230‐239 28 1255 980‐1470 900‐1860 240‐249 36 1435 1200‐1790 1080‐1950 250‐259 37 1580 1290‐1925 1180‐2260 260‐269 56 1835 1490‐2190 1340‐2400 270‐279 89 2000 1640‐2320 1390‐2620 280‐289 134 2265 1920‐2660 1530‐2910 290‐299 219 2530 2130‐2900 1820‐3100 300‐309 350 2685 2340‐3080 2010‐3420 310‐319 387 2850 2470‐3290 2110‐3650 320‐329 484 3060 2700‐3470 2350‐3770 330‐339 439 3260 2880‐3700 2570‐3980 340‐349 423 3380 3040‐3860 2670‐4240 350‐359 314 3615 3240‐4040 2890‐4460 360‐369 245 3750 3330‐1190 3020‐4610 370‐379 117 3840 3480‐4360 3180‐4790 380‐389 66 4140 3660‐4640 3470‐4820 390‐399 35 4290 3665‐4675 3640‐5000