Should we use the “ellipse” or “two diameters” method to measure fetal head and abdominal circumferences?

Objectives: To assess the reproducibility, throughout gestation, of the two most commonly used methods to measure fetal head (HC) and abdominal (AC) circumferences, namely fitting of an “ellipse” and measuring “two diameters”. Methods: Women participating in the INTERGROWTH-21st Project had serial ultrasound scans every 5 ± 1 weeks from 9-14 weeks of gestation until delivery. At each visit, HC and AC were measured in triplicate in two ways: using the ellipse facility (HCellipse and ACellipse) and by measuring two perpendicular diameters (biparietal and occipitofrontal diameter of the head, and transverse and antero-posterior diameters of the abdomen), to calculate HCcalc and ACcalc, respectively. To assess the intra-observer reproducibility, the standard deviation (SD) for each of the triplicate measurements was compared between the two methods (paired t-test). Inter-observer reproducibility of caliper replacement was performed on a random sub-sample of 10% of the scans. Results: A total of 20,029 scans provided measurements using both methods. The intra-observer variability based on the triplicate measurements for the two methods was not significantly different (HCellipse vs HCcalc p = 0.41; ACellipse vs ACcalc p = 0.36). The mean systematic differences (95% limits of agreement) for the inter-observer variability were all similar (HCellipse: −2.4 (6.6, −11.5); HCcalc: −1.9 (7.7, −11.6); ACellipse: 0.10 (11.6, −11.4); ACcalc: 0.06 (11.8, −12.0) mm). Conclusions: There are minimal differences in measurement variability between the ellipse and two diameters methods, which suggests the two methods are equally reproducible. There appears to be no advantage in using one method over the other in clinical practice