What factors contribute to inaccuracies in fetal ultrasound size measurements during the third trimester of pregnancy?

From the Guidelines

Inaccuracies in third-trimester ultrasound size measurements can be attributed to various factors, including fetal position, maternal factors, technical factors, biological variations, and multiple gestations, which can lead to an error margin of ±10-15%. Several factors can lead to inaccuracies in third-trimester ultrasound size measurements. Fetal position plays a significant role, as suboptimal positioning can prevent proper visualization and measurement of structures. Maternal factors such as obesity or excessive amniotic fluid (polyhydramnios) can reduce image quality by increasing the distance between the ultrasound probe and the fetus. Technical factors including equipment quality, sonographer experience, and measurement technique variability contribute to inconsistencies. Biological variations in fetal growth patterns mean that standard growth charts may not accurately represent all populations, leading to misclassification of normal versus abnormal growth, as highlighted in the Society for Maternal-Fetal Medicine consult series #52 1. As pregnancy progresses, the accuracy of ultrasound measurements typically decreases due to the fetus becoming more crowded in the uterus, with less amniotic fluid relative to fetal size, and increased calcification of fetal bones that can create acoustic shadowing. Multiple gestations present additional challenges as fetuses may overlap, making individual measurements difficult. Some key points to consider when interpreting ultrasound measurements in the third trimester include:

• The use of population-based fetal growth references, such as Hadlock, to determine fetal weight percentiles 1
• The importance of serial umbilical artery Doppler assessment to assess for deterioration in fetal growth restriction 1
• The consideration of delivery at 37 weeks of gestation in pregnancies with fetal growth restriction and an umbilical artery Doppler waveform with decreased diastolic flow but without absent/reversed end-diastolic velocity or with severe fetal growth restriction with estimated fetal weight less than the third percentile 1 These limitations explain why third-trimester ultrasound measurements often have an error margin of ±10-15%, which is important to consider when making clinical decisions based on these measurements.

From the Research

Factors Affecting Ultrasound Size Measurements

• Operator error and variability in measurement techniques can lead to inaccuracies in ultrasound size measurements 2, 3
• The timing of the ultrasound measurement can also impact accuracy, with studies suggesting that measurements taken between 34.0 and 36.9 weeks' gestation may be more accurate than those taken later in gestation 3
• Fetal macrosomia, or a fetal weight above the 90th percentile, can be difficult to accurately measure using ultrasound, with one study finding that estimated fetal weights exceeded birth weights in 77% of cases 2
• The use of different formulas and techniques for estimating fetal weight can also lead to variations in measurement accuracy 2, 3
• Maternal factors, such as body mass index and diabetes, can also impact the accuracy of ultrasound size measurements 4, 5

Limitations of Ultrasound Measurements

• Ultrasound measurements may not always accurately reflect fetal size, particularly in cases of fetal macrosomia or growth restriction 2, 4
• The accuracy of ultrasound measurements can be affected by a range of factors, including operator error, fetal position, and maternal body habitus 2, 3
• Studies have shown that routine third-trimester ultrasounds may not decrease the rate of perinatal death compared with serial fundal height measurements in low-risk pregnancies 6
• The use of ultrasound measurements to predict birth weight and identify fetal growth restriction or macrosomia may be limited by the accuracy of the measurements themselves 2, 3