From the Research
Ultrasonographic measurement in obstetrics relies on several key principles to ensure accurate fetal assessment, including standardization of measurement techniques, timing of measurements, and quality control, as emphasized by the importance of proper calibration and training 1. The primary principle is standardization of measurement techniques, which involves obtaining images in specific planes and using consistent anatomical landmarks. For example, biparietal diameter (BPD) should be measured at the level of the thalami and cavum septi pellucidi in an axial plane perpendicular to the falx. Timing of measurements is also crucial, with first trimester dating being most accurate, ideally between 8-14 weeks using crown-rump length, as this period provides the most reliable estimation of gestational age 2. Multiple parameters should be measured for comprehensive assessment, including:
- Head circumference
- Abdominal circumference
- Femur length in the second and third trimesters Quality control is essential, requiring:
- Proper calibration of equipment
- Adequate training of sonographers
- Obtaining multiple measurements when possible to minimize error Interpretation must consider biological variation, with measurements compared to established growth charts appropriate for the population. Serial measurements are more valuable than single assessments for monitoring fetal growth, typically spaced 2-4 weeks apart when tracking growth concerns, and the combined use of Doppler velocity wave form analysis of fetal vessels and the ultrasonic estimation of fetal weight (or abdominal circumference) appears to be the best method of both identification and evaluation of intrauterine growth restriction (IUGR) 2. Proper technique includes:
- Using the correct transducer frequency (higher for early pregnancy, lower for later stages)
- Maintaining the ultrasound beam perpendicular to the structure being measured
- Using electronic calipers placed at specific anatomical boundaries These principles ensure reliable measurements for accurate dating, growth assessment, and detection of fetal anomalies throughout pregnancy, and it is the role of science to show whether any bioeffects of ultrasound may be harmful, as well as to educate end users to assure patients' safety 1.