GASTROSCHISIS: A NEW FETAL WEIGHT FORMULA TO PREDICT BIRTH WEIGHT KIRAN TAM TAM1, LAURA BUFKIN2, JAMES BOFILL2 1Maternal Fetal Medicine, Baylor College of Medicine / Texas Children’s Hospital, Houston, TX, USA 2Maternal Fetal Medicine, University of Mississippi Medical Center, Jackson, Mississippi, USA OBJECTIVE: To derive a sonographic fetal weight formula for fetuses with gastroschisis and evaluate it against the Shepard (1982) formula in the prediction of birth weight (BW). STUDY DESIGN: Retrospective review of 115 cases of gastroschisis after 2000. A fetal weight formula was derived by multiple linear regression analysis using BW and sonographic fetal biometry (obtained within 7 days of birth) of 41 infants with gastroschisis. This formula and the Shepard (1982) formula were then used to calculate the estimated fetal weight (EFW) of 25 other infants with gastroschisis that were delivered within 2 weeks of fetal biometry measurements. The accuracy of the two formulas to predict the actual birth weight (ABW) in these 25 infants was evaluated. RESULTS: Head circumference (HC; P < 0.001) and abdominal circumference (AC; P = 0.001) measured in centimeters were significant predictors of BW in grams (BW = -4079.683 + (134.430 * HC) + (80.717 * AC). The correlation coefficient and the standard error of estimate was 0.8 and 247 grams respectively. The mean difference in EFW and ABW (+/- standard error of the mean) for this and Shepard's formula was 212 +/- 56 grams and 392 +/- 54 grams respectively (P = 0.025). CONCLUSION: The sonographic fetal weight formula derived from the biometry of fetuses with gastroschisis is more accurate in the prediction of BW of infants with this anomaly than the Shepard (1982) formula. BACKGROUND: We previously reported that the prognosis for infants with gastroschisis was dependent on birth weight (BW) and that this weight was most accurately determined by the Shepard (1982) formula (Log10 BW = -1.7492 + 0.166 (BPD) + 0.046 (AC) -(2.646 [(AC) X (BPD)] /100))1,2. However, we hypothesized that a sonographic fetal weight formula derived from the biometry of fetuses with gastroschisis may be more accurate in the prediction of BW for infants with this anomaly. METHODS: 115 cases of gastroschisis after 2000 were reviewed at our institution. Fetal weight formulas were derived by multiple linear regression analysis using BW and sonographic fetal biometry (obtained within 7 days of birth) of 41 infants with gastroschisis. The variance inflation factor, a measure of collinearity between variables in regression analysis, was as expected high for head circumference (HC), biparietal diameter (BPD) and occipito-frontal diameter (OFD). Previous studies have shown that the deviations of BW estimates from the actual BW are the smallest when HC is used in such analysis3. In addition, HC, a function of both BPD and OFD, has been proposed to be a better head-size modulus than the commonly used BPD2. A total of seven fetal weight formulas were derived using the birth weights of these 41 infants and various combinations of their sonographic biometry parameters like abdominal circumference (AC), HC and femur length (FL). These formulas and the Shepard (1982) formula were then used to calculate the estimated fetal weight (EFW) of 25 other infants with gastroschisis that were delivered within 2 weeks of fetal biometry measurements. The accuracy of the two formulas to predict the actual birth weight (ABW) in these 25 infants was evaluated. RESULTS: All formulas on average underestimated BW. Fetal weight formula #2 was the most accurate of the seven formulas in determining the BW of infants with gastroschisis. The agreement between ABW and the weight estimated by formula # 2 was significantly better than that estimated by the Shepard (1982) formula. CONCLUSION: The sonographic fetal weight formula derived from the biometry of fetuses with gastroschisis is more accurate in the prediction of BW of infants with this anomaly than the Shepard (1982) formula. REFERENCES 1Tam Tam KB et al. Am J. Perinatol. 2011;28(9):689-94. 2Shepard MJ et al. Am J Obstet Gynecol 1982; 142:47. 3Jordaan HV. J Clin Ultrasound. 1983;11(2):59-66. kbtam@bcm.edu