Are ultrasound foetal circumference measurement methods interchangeable?

Area of Science:

• Obstetrics and Gynecology
• Medical Imaging
• Fetal Biometry

Background:

• Ultrasound is crucial for fetal biometry, with ellipse fitting and derived measurements commonly used for circumferences.
• These methods are often assumed to be equivalent, despite potential differences impacting clinical practice.
• Accurate fetal measurements are vital for monitoring growth and estimating fetal weight.

Purpose of the Study:

• To investigate and quantify the differences between ellipse fitting and derived fetal circumference measurements in clinical settings.
• To assess the clinical implications of these differences on estimated fetal weight calculations.
• To provide guidance on appropriate method selection based on existing charting standards.

Main Methods:

• Retrospective analysis of fetal head and abdominal circumferences measured via ellipse fitting.
• Derivation of circumferences using a standard formula from anterior-posterior and transverse diameters.
• Comparison of ellipse fitted and derived measurements using Bland-Altman plots, stratified by gestational age groups.

Main Results:

• Small systematic differences were observed between ellipse fitted and derived measurements.
• Random differences varied significantly across gestational age groups, with wider confidence intervals in later scans.
• 95% confidence intervals for head circumference ranged from ±3% to ±6%, abdominal circumference from ±5% to ±6%, and estimated fetal weight from ±5% to ±12%.

Conclusions:

• Ellipse fitting and derived fetal circumference measurement methods are not interchangeable.
• The choice of method significantly impacts measurement accuracy and estimated fetal weight.
• The derived method should be preferentially used when referencing charts based on this methodology for fetal size, growth, and weight estimation.