OP01.06: Accuracy and precision of sonographic fetal weight estimation in specific subgroups: important knowledge in clinical decision making

Fetal growth assessment by ultrasound is an essential part of decision making in modern obstetric care. The aim of this study was to evaluate accuracy and precision of fetal weight estimation in specific subgroups. The study population consisted of 31,521 singleton pregnancies delivered at 22+0 gestational weeks or later, with an ultrasound estimated fetal weight (EFW) performed within two days before delivery, registered in the Swedish Pregnancy Register between 2014 and 2021. Fetal biometric ultrasound measurements were used to calculate EFW according to the formula by Persson and Weldner. Systematic error (mean percentage error, MPE) was calculated and stratified by maternal BMI-categories, fetal sex, fetal position and EFW (estimated small, appropriate or large for gestational age; SGA, AGA or LGA), all in combination with gestational age. For the whole study population, MPE was −2.7 (SD 8.9). MPE was largest for the most preterm fetuses estimated as severe SGA (< 24 weeks and EFW < 3rd percentile, MPE -13.9 ± 14.2 SD). While actual birthweight was underestimated in all gestational weeks in fetuses estimated as severe SGA, actual birthweight was generally overestimated in fetuses estimated as severe LGA (> 97th percentile; see figure 1). When stratified by maternal BMI-categories, fetal sex and fetal position, minor differences in MPE were found. The systematic underestimation of actual birthweight was largest in extreme preterm estimated SGA-fetuses, while actual birthweight was generally overestimated in fetuses estimated as severe LGA. Knowledge about systematic errors in the fetal weight estimation formula used is of utmost importance in clinical decision making. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.