The BPD remains the standard against which other parameters of gestational age assessment are compared.

The anatomical landmarks used to ensure the accuracy and reproducibility of the measurement include: 1) a midline falx, 2) the thalami symmetrically positioned on either side of the falx, 3) visualization of the Septum Pellucidum at one third the frontooccipital distance. A wrong measurment plane can produce errors up to 20mm !

A leading edge to leading edge measurement or a middle-to-middle measurement are both acceptable.

The BPD should be measured as early as possible after 13 weeks for dating.

The BPD can be smaller (and sometimes much smaller than is expected) in fetuses with flatter heads. If the head really looks flat on the scan, check the head circumference and record the findings. If the value is within the normal range, then most likely the discrepancy is due to a flat head. Monitor the growth of the fetal head with the circumference from then on. The ceplalic index will also be useful. If the value is below 74, the head is consider excessively flat. In cases of microcephalics, the BPD is usually at least 2 SD below the mean.

The BPD is useful for dating a pregnancy and in estimating intrauterine fetal weight in weight equation, other than that itís value is limited and can sometimes be misleading in the assessment of growth in the fetus if it is the sole measurement taken for interpretation.

Joseph Woo.

Back to Obstetric ultrasound