Fig 1.

Transverse image of fetal head suitable for BPD, OFD and HC measurements (A) BPD = distance measured at right angle between tips of the solid arrows (depicting outer edge to inner edge approach). Occipito-frontal diameter (OFD) = the distance between the tips of the open arrows. The head circumference was measured by tracing the distance round the skull. (B). Diagram of the transverse image of the fetal skull showing the thalami (large arrow), midline unbroken echo of the falx cerebri (curved arrow), and double echogenic lines (open arrow) of cavum septi pellucidi.