Limited clinical utility of midtrimester fetal morphometric percentile rankings in screening for birth weight abnormalities

OBJECTIVE: Our purpose was to determine whether midtrimester fetal ultrasonographic morphometric percentile rankings are sensitive screening tests for preterm labor or birth weight abnormalities. STUDY DESIGN: Stepwise multiple regression and chi 2 analysis were used to identify midtrimester fetal measurements predicting birth weight and gestational age. Receiver-operator characteristics curves were used to evaluate abdominal circumference percentiles as a test for large-for-gestational-age and small-for-gestational-age infants. RESULTS: Extremes in abdominal circumference and head measurement percentiles were associated with large- and small-for gestational-age infants but not with preterm delivery. Abdominal circumference predicted birth weight in regression analysis; however, receiver-operator characteristic curves showed abdominal circumference percentiles to be poor screening tests for large- or small-for-gestational-age infants. The positive predictive value of 10th and 90th abdominal circumference percentiles for small- and large-for-gestational-age infants was < 20%. CONCLUSION: Midtrimester percentile rankings offer no clear benefit in targeting fetuses with potential birth weight abnormalities or risk of preterm delivery and may provide clinically misleading information.