Estimated fetal weights
At the BC Women’s Hospital, ultrasound assessment of fetal size is performed by sonographers certified in obstetrical ultrasound and verified by a Maternal Fetal Medicine or Radiology physician. Measurements are taken twice and the average of the two is used. The Hadlock3 formula, which combines head circumference, abdominal circumference, and femur length measurements was used to calculate estimated fetal weights. The most recent measurement before delivery was used. The accuracy of estimated fetal weights at our institution has previously been shown to be + 3.3% (SD 12.1), with 67% of fetuses having an estimated fetal weight within ±10% of their actual birthweight. Estimated fetal weights were converted into gestational age-specific percentiles using the previously published INTERGROWTH 21st and WHO standard. We used recommended methods to calculate exact percentiles for weights falling between published percentiles on the WHO chart.