·Abdominal circumference- Is used: (1) to assess fetal growth; (2) to evaluate any disproportion between head and body size; (3) to evaluate appropriate fetal growth over a period of time.
·Head circumference- The head circumference may be measured directly by marking the margins of the skull or indirectly by the formula:
Head circumference = (BPD +OFD) x 1.62.
·Multiple parameters- (BPD, FL, HC, etc.) Averaging the results of multiple parameters is not universally accepted. Proponents state it results in approximately 25% increase in accuracy in predicting gestational age. Many other parameters have been reported to be reliable indicators of gestational age. Their main limiting factor in later pregnancy is the increasing importance of genetic variation and alteration in growth, therefore the measurement cannot accurately predict gestational age after 24 weeks.
·Fetal weight- Analysis of fetal weight may be performed using standard fetal measurement (AC, HC, FL, BPD, etc.), either as single parameters (AC) or in combination of up to 4.The use of AC gives accuracy of±160 g/kg fetal weight. Combining the use of BPD and AC increases accuracy to ±106 g/kg but adding further parameters improves accuracy by no more than 1%. The fetal weight is estimated by measuring the chosen parameters and comparing them with standard charts for weight.
Unfortunately all the formulae tend to overestimate the weight of the low birthweight fetus, although most have an overall accuracy of within ±10%. In future 3D ultrasound may give us better estimates.