Accurate prediction of term birth weight from prospectively measurable maternal characteristics.

Objective: To determine whether accurate prediction of term birth weight is possible based on maternal characteristics routinely measured remote from term, and to compare this technique to more expensive methods requiring obstetrical ultrasound examinations.Methods: Two hundred fifty-nine normal, non-smoking, non-diabetic Caucasian gravidas with uncomplicated term gestations were studied. Seven maternal characteristics (age, parity, height, weight, level of obesity, rate of pregnancy weight gain, and glucose screening test result) and two fetal characteristics (fetal gender and length of gestation) were evaluated alone and in combination for their predictive values in determining birth weight. A cross-validated split-sample multiple regression analysis was used to determine which combinations of these variables were significant and a birth weight prediction equation was developed. Predictive accuracy was assessed using a jackknifing procedure, and results were compared to similar types of birthweight predictions obtained both via previous algorithms based upon maternal characteristics and those developed for use with fetal ultrasonographic biometric data.Results: Significant predictors of term birth weight were gestational age, fetal gender, parity, maternal height, maternal weight, and third trimester maternal weight gain rate. These prospectively measurable variables explained 33% of the variance in birth weight and predicted birth weight to within 10.8%. These results were compared to those obtained from other previously published algorithms and were more accurate than all others derived from either maternal characteristics or fetal ultrasonographic data. Our term birth weight prediction equation is: birth weight (g)=gestational age (days)x[9.40+0.255xgender+ 0.000232xheight (cm)xmaternal weight at 26 wk (kg)+ 4.89x3rd trimester weight gain rate (kg/d)x(parity+1)]where: gender=-1 for females;+1 for males;0 for unknown gender gestational age=conceptual age (days)+14Conclusion: Term birth weight can be accurately predicted using routinely measurable maternal characteristics. Birth weight estimates using our equation are both prospectively derivable starting from the end of the second trimester and more accurate than any previously devised algorithms, including methods based upon ultrasonographic fetal biometric data.