Sonographic fetal weight estimation percentiles should be interpreted with caution in the second trimester

To describe the limitations of sonographic estimation of fetal weight (EFW) percentiles during the second trimester; compare the accuracy of EFW percentiles to that of single fetal biometric indice; and identify the gestational age threshold beyond which the sonographic diagnosis of fetal growth restriction (FGR) becomes more reliable.

We described the limitations of EFW by simulation using the 1991 Hadlock. Then, we illustrated these limitations using data from a cohort of singleton pregnancies at a single tertiary center (2014-2023). The primary predictor was EFW percentile, and the primary outcome was birth weight <10th percentile for gestational age. The diagnostic accuracy was described using the area under the receiver-operator characteristic curve, detection rate, false positive rate, and positive and negative predictive values.

Less than 25 weeks of gestation, EFW percentiles are highly sensitive to minor dating or sonographic measurement errors. Abdominal circumference (AC) percentile is less sensitive to these errors and may be a more reliable measure. Using a cohort of 29 608 patients (76 378 ultrasound examinations), we confirmed that the positive predictive value (PPV) of EFW percentile for birth weight <10th percentile decline sharply before 25 weeks of gestation. In addition, before 25 weeks of gestation, AC <10th percentile has a higher PPV for birth weight <10th percentile than EFW <10th percentile (45% vs. 41% at 15^0/7-19^6/7 weeks, and 61% vs. 56% at 20^0/7-24^6/7 weeks).

Sonographic EFW percentiles are less reliable before 25 weeks than later in pregnancy, and should be interpreted with caution during this gestational age period.