From the Guidelines
In women with low BMI during third trimester ultrasounds, fetal size measurements tend to be more accurate compared to women with higher BMI, but may be subject to a slight overestimation of fetal weight due to decreased soft tissue between the ultrasound probe and the fetus. According to the Society for Maternal-Fetal Medicine consult series #52 1, the use of population-based fetal growth references, such as Hadlock, is recommended in determining fetal weight percentiles. However, it is essential to consider the potential for overestimation in women with low BMI, as this may affect clinical decision-making regarding delivery timing or interventions for suspected growth restriction.
Some key points to consider when measuring fetal size in women with low BMI include:
- Improved ultrasound visualization due to lower BMI, which reduces measurement errors 1
- Potential for slight overestimation of fetal weight, typically around 5-10%, due to decreased soft tissue between the ultrasound probe and the fetus
- Better visualization of fetal anatomy, more precise biometric measurements, and more accurate amniotic fluid assessment in low BMI women
- The need for clinicians to be aware of this potential for slight overestimation when interpreting growth percentiles and estimated fetal weight in women with low BMI, particularly when making decisions about interventions for suspected growth abnormalities.
It is crucial to prioritize the most recent and highest quality study, which in this case is the Society for Maternal-Fetal Medicine consult series #52 1, to ensure that clinical decisions are based on the best available evidence. By considering the potential for overestimation in women with low BMI, clinicians can make more informed decisions regarding fetal growth restriction and delivery timing.
From the Research
Inaccuracies in Measuring Fetal Size on 3rd Trimester Ultrasound for Women with Low BMI
- There is no direct evidence in the provided studies that specifically addresses the inaccuracies in measuring fetal size on 3rd trimester ultrasound for women with low BMI 2, 3, 4, 5, 6.
- However, the studies suggest that fetal biometric parameters such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur diaphysis length (FL) can be used to estimate fetal weight (EFW) using various formulae 2.
- Accurate estimation of gestational age is a prerequisite for determining whether fetal size is appropriate-for-gestational age (AGA) 2.
- Maternal factors such as BMI may affect the accuracy of fetal size measurements, but this is not explicitly addressed in the provided studies.
- The studies focus on the management of fetal growth restriction (FGR) and the use of ultrasound and Doppler velocimetry to assess fetal well-being 3, 4, 5, 6.
- The role of maternal BMI in fetal size measurement is not discussed in the provided evidence, and therefore, it is unclear how low BMI may affect the accuracy of fetal size measurements on 3rd trimester ultrasound.