OC02.07: Ultrasound predicted birthweight and identification of growth abnormalities in severely obese women: a national cohort study

To evaluate third trimester ultrasound predicted birthweight in severely obese women, with focus on identifying small and large for gestational age (SGA and LGA) fetuses, and to determine the optimal gestational age (GA) for the ultrasound examination. Danish national cohort study of 19,504 pregnancies (2009-2017) with BMI ≥35 kg/m2, and at least one third trimester ultrasound estimated fetal weight (EFW). Exclusion criteria included conditions affecting intrauterine fetal growth, such as pre-eclampsia and diabetes. The ultrasound predicted birthweight was determined by ultrasound EFW, calculated by Hadlock's formula and referencing an intrauterine growth curve. SGA and LGA were defined as the 10th and 97,7th percentile, respectively. The population was divided into three groups based on GA at ultrasound examination: group 1 (28+0-34+6), Group 2 (35+0-37+6) and Group 3 (>38+0). Subanalyses were performed on pregnancies with two or more ultrasounds at different GA. The study included 7,231 pregnancies, of which 1,396 had two or more ultrasound examinations. The absolute birthweight error (gram) mean ± SD was 3g (SD 375), with a 95% CI. Within GA subgroups errors were -136g (SD 411), 21g (SD 365), and 69g (SD 364) for Groups 1, 2, and 3, respectively. The area under curve (AUC) for predicting LGA was 0,87 in Group 2 and 0,88 in Group 3, both significantly higher than Group 1's AUC of 0,81 (p < 0.02). For predicting SGA, the AUC was 0,95 in Group 3, significantly higher than Group 1 and Group 2's AUC of 0,84 and 0,92, respectively. During the third trimester, ultrasound has the highest predictive accuracy for identifying LGA after the 35th week and SGA after the 38th week. Ultrasound EFW assessment after 35 weeks of gestation reduces birthweight prediction errors and enhances the accuracy of identifying both LGA and SGA fetuses.