Fig. 1.

The biometric measurements of the fetal head. The BPD measurement is taken on the outer border of the parietal bones (outer to outer) at the widest part of skull. The OFD is measured between the outer border of the occipital and frontal edges of the skull at the point of the midline (outer to outer) across the longest part of skull. The HC is the HC calculated from the formula $\mathrm{HC}=\pi (\mathrm{BPD}+\mathrm{OFD})/2$.²