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.