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

OBJECTIVE: To determine whether accurate prediction of individual term birth weight is possible based on maternal characteristics routinely measured remote from term in healthy women.

STUDY DESIGN: Two hundred sixty-two nonsmoking, nondiabetic, white gravidas with uncomplicated term gestations were studied. A cross-validated, split-sample multiple regression analysis was performed to evaluate the predictive value of seven maternal characteristics and two fetal characteristics to identify an optimal combination for accurately estimating birth weight. Fifty-nine terms were assessed for predictive accuracy.

RESULTS: Significant predictors of term birth weight were gestational age, parity, fetal sex, maternal height, maternal weight and third-trimester maternal weight gain rate. Combinations of these prospectively measurable variables explained 33% of the variance in birth weight and predicted birth weight to within +/- 267 g (+/- 7.6% of individual birth weight). Term fetal macrosomia was predicted with 80% sensitivity using a prediction cutoff of 3,550 g.

CONCLUSION: Individual term birth weight can be accurately predicted in normal gravidas using routinely measurable maternal characteristics. Birth weight estimates using our equation are both prospectively derivable from the beginning of the third trimester and more accurate than any previously devised algorithms, including those that incorporate fetal ultrasonographic data. Our equation can also identify pregnancies at risk for fetal macrosomia so that the timing and mode of delivery may be prospectively modified to minimize peripartum risks to both fetus and mother.