Thus, a value that is above 50% indicates that the measurement is above average.
The BPD and OFD are measured on a transverse axial section of the fetal head which includes the falx cerebri anteriorly and posteriorly, the cavum septum pellucidum anteriorly in the midline and the thalami.
No formula for estimating the fetal 'birth weight' has achieved an accuracy which enables us to recommend its use.
It should be noted that errors of about 10% are reported and that even at this level of accuracy it is disappointing.
This is preferable to just averaging all values to arrive at an estimated gestational age.
The earliest measurement of gestational age taken in pregnancy should usually be accepted as the definitive assessment, subsequent examinations reflecting only fetal growth in the intervening period.
If measurements taken after the first trimester are within one week of the gestational assessment taken from menstrual dating then the ultrasound assessment of gestational age confirms the menstrual dates.
The wide normal range of BPD in late pregnancy must be appreciated.
It is not expected that BPD be used to assess gestation late in pregnancy.