Sonographically estimated fetal weight percentile as a predictor of preterm delivery

To evaluate the association between relative growth restriction and preterm birth.

Pregnant women referred for sonographic fetal weight assessments between 24 and 34 weeks of gestation were studied for gestational age at delivery. If a patient underwent more than one study, only the last one was considered. Patients with delivery induced iatrogenically or with abnormal growth patterns due to known pathology, such as maternal diabetes or fetal congenital anomaly, were excluded. A gestational age of 37 weeks or less was considered preterm and a gestational age of more than 37 weeks at delivery was considered term. Fetal weight estimation was obtained by Hadlock's formula based on biparietal diameter, femur length, and head and abdominal circumferences. The estimated fetal weight percentile was computed according to William's tables. Mean gestational age and incidence of preterm delivery for each fetal weight percentile between 1 and 100, at increments of 10, were calculated. The mean estimated fetal weight percentile, biparietal diameter, femur length, head circumferences and abdominal circumferences of preterm and term patients were compared.

Among the 419 patients who met the inclusion criteria, duration of gestation was significantly shorter in fetuses with low estimated fetal weight percentile. The risk of preterm birth was 49% in fetuses of less than the 40th birth-weight percentile compared with a risk of 20% in fetuses of more than the 40th birth-weight percentile, representing a relative risk of 2.3. Individual fetal measurements indicate a head-sparing effect in the preterm group.

Sonographically estimated fetal weight percentile measured between 24 and 34 weeks' gestation may be used as an additional and individually pertinent predictor of preterm birth.