Biparietal diameter measurements using the outer-to-outer versus outer-to-inner measurement: A question of pedantry?

Objectives: To compare two methods of measuring fetal biparietal diameter (BPD) - outer-to-inner (BPDoi) vs. outer-to-outer (BPDoo) calliper placement - and to compare the differences in EFW calculated using the Hadlock 4 formula and other common EFW formulae.

Methods: A total of 543 fetuses underwent a single ultrasound prospectively performed by 40 sonographers between 14 and 40 weeks of gestation, taking into account the intra- and inter-observer variability. The measurements for each fetus consisted of BPDoi and BPDoo, and EFW is calculated from HC, AC and FL measurements. The difference between BPDoo and BPDoi was estimated, and this difference was compared with gestational age using linear regression. Translational equations that allow interconversion of the two parameters were derived. EFW calculated from four different formulae using various combinations of biometric measurements was compared.

Results: The difference between BPDoi and BPDoo increases with gestational age, although this difference was small. For BPDoo, the regression equation is BPDoo = 0.555934 + 1.027318 × BPDoi. Similarly, for BPDoi, the regression equation is BPDoi = -0.403458 + 0.9714153 × BPDoo. There is a minimal difference in the EFW calculated from the four formulae, except for gestations prior to 27-28 weeks. EFW derived from INTERGROWTH-21st formulae plot is higher than that from Hadlock 3 or Hadlock 4 before 27-28 weeks.

Conclusions: Although the absolute difference between BPDoo and BPDoi increased across gestational age, this difference was small. The method of BPD measurement should follow that as prescribed in the EFW equation used in the local context. Estimation of fetal weight using Hadlock 3, Hadlock 4 and INTERGROWTH-21st is similar, with slight differences at gestations less than 27-28 weeks.