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

Abstract Objectives To compare two methods of measuring fetal biparietal diameter ( BPD ) – outer‐to‐inner ( BPD oi) vs. outer‐to‐outer ( BPD oo) 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 BPD oi and BPD oo, and EFW is calculated from HC , AC and FL measurements. The difference between BPD oo and BPD oi 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 BPD oi and BPD oo increases with gestational age, although this difference was small. For BPD oo, the regression equation is BPD oo = 0.555934 + 1.027318 × BPD oi. Similarly, for BPD oi, the regression equation is BPD oi = −0.403458 + 0.9714153 × BPD oo. 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 BPD oo and BPD oi 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.