Which is best for fetal head biometry: the transthalamic or transventricular plane?

Objectives: There is ongoing disagreement in the literature as to whether fetal head biometry should be measured in the transthalamic (TT) or the transventricular (TV) plane; and whether HC should be measured using an ellipse or by measuring BPD / OFD. In this study we assess both these factors. Methods: 100 fetuses were prospectively scanned between 16 and 40 weeks as part of the INTERGROWTH-21ST project at the University of Oxford. The biparietal diameter (BPD), occipitofrontal diameter (OFD) and head circumference (HC) were obtained for the TT and the TV plane. The biometry was re-measured by replacing callipers. Differences were expressed as a percentage of the fetal measurements to account for gestational age effects. Bland-Altman plots test were performed. Results: The mean systematic bias between observers was \u3c 1% (Table 1). All measurements were repeatable to within 16% in 95% of cases. Neither TV nor TT plane was consistently more reproducible than the other. Overall, around half the variability was due to calliper replacement. The measurement of HC using an ellipse was the most reproducible. Conclusions: The variability introduced by measuring in the two different planes (TT vs. TV) appears lower than the variability between sonographers. The HC by using the ellipse facility should be preferred to calculating from BPD and OFD