How to measure the BPD

The Hadlock-formula is being widely used for the estimation of fetal weight. Hadlock explained the reasons behind the choice of the plane section for sonographic measurement of the bi-parieral diameter (BPD).

  1. The fetal head should occupy at least 30% of the image and should appear as an ovoid structure, while the midline echo or falx cerebri should be vertical to the ultrasound beam.
  2. Acquire a symmetrical axial plane of section with the widest transverse and frontal-occipital diamensions of the fetal head where the falx cerebri, the cavum septi pellucidi, both thalami and both cerebral peduncles are visible (Sarris et al, 2011).
  3. Place the calipers at the outer surface of the skull side near the transducer and at the inner margin of the opposite skull side. While outer-to-outer measurements have also been described, we suggest the use of the outer-inner method because this was the method used by Hadlock (Hadlock et al, 1982).
Biparietal diameter of the fetal head


  • Sarris, I., Ioannou, C., Dighe, M., Mitidieri, A., Oberto, M., Qingqing, W., Shah, J., Sohoni, S., Al Zidjali, W., Hoch, L., Altman, D. G., Papageorghiou, A. T. and for the International Fetal and Newborn Growth Consortium for the 21st Century (INTERGROWTH-21st) (2011), Standardization of fetal ultrasound biometry measurements: improving the quality and consistency of measurements. Ultrasound Obstet Gynecol, 38: 681–687. doi:10.1002/uog.8997
  • Hadlock FP, Deter RL, Harrist RB, et al. Fetal biparietal diameter: Rational choice of plane section for sonographic measurement. AJR Am J Roentgenol. 1982;138:871-874.

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