When an ultrasound provides an Estimated Fetal Weight (EFW), it offers a projection of the baby’s size. It is important to understand that EFW is a mathematical projection, not a direct measurement of mass, meaning it inherently carries a degree of uncertainty. This estimated figure serves as a tool for clinical assessment, providing a benchmark for monitoring growth.
How Estimated Fetal Weight is Calculated
The process of determining EFW begins with the sonographer taking specific linear measurements of the fetus during the ultrasound examination. These measurements, known as fetal biometry, focus on anatomical structures that correlate with overall fetal size. The four primary measurements used for this calculation are the Biparietal Diameter (BPD), the Head Circumference (HC), the Abdominal Circumference (AC), and the Femur Length (FL).
The measurements are input into a computerized regression equation, most commonly one of the Hadlock formulas. These formulas are mathematical models developed by comparing ultrasound measurements to the actual birth weights of thousands of infants. The formulas translate the two-dimensional measurements into a three-dimensional weight estimate, typically expressed in grams.
The Typical Margin of Error and Accuracy
The accuracy of an EFW measurement typically falls within a margin of error of plus or minus 10% to 15% of the actual birth weight. This range is the standard statistical error for measurements taken in utero. For an EFW to be considered accurate, the baby’s actual weight at birth should fall within this percentage range of the estimate.
If an EFW is reported as 7 pounds (3,175 grams), the actual birth weight could reasonably range from about 6 pounds, 1 ounce to 8 pounds, 1 ounce, assuming a 15% margin of error. This variance is due to the challenge of estimating a three-dimensional body mass using two-dimensional images. Accuracy tends to be lower for fetuses at the extremes of size, such as those who are very small or very large for their gestational age. Estimates for babies suspected of having macrosomia (excessive size) are often overestimated by the ultrasound.
Clinical Reasons for Estimating Fetal Weight
Medical professionals rely on EFW for its utility in identifying potential growth issues that require intervention, not for its absolute precision. The estimate is primarily used to screen for two conditions that affect perinatal outcomes: Fetal Growth Restriction (FGR) and suspected macrosomia. A fetus with an EFW below the 10th percentile for its gestational age is identified as being at risk for FGR.
EFW influences decisions regarding the timing and mode of delivery. For a fetus diagnosed with FGR, the estimate, often combined with Doppler flow studies, helps determine the optimal time for delivery. In cases of suspected macrosomia, EFW is a factor in counseling parents about delivery options. If the EFW exceeds 4,500 grams (with diabetes) or 5,000 grams (without diabetes), a prophylactic cesarean delivery may be considered to mitigate the risk of complications like shoulder dystocia. Suspicion of a large fetus based on EFW can also increase the likelihood of a cesarean delivery.
Factors That Increase or Decrease Accuracy
Several factors can modify the 10% to 15% error rate, either improving or diminishing the accuracy of the EFW. The time interval between the ultrasound and delivery is a significant factor, as estimates performed within seven days of birth are more accurate than those taken earlier. Fetal position can also create measurement challenges; for instance, a fetal head deeply engaged in the pelvis near term can obscure the necessary landmarks for accurate head and biparietal diameter measurements.
Maternal Body Mass Index (BMI) affects accuracy, as a higher BMI makes it more difficult for ultrasound waves to penetrate and obtain clear images of the fetal structures. The skill and experience of the sonographer also play a direct part in the quality of the raw measurements used in the calculation. Sonographer variability in measurement technique is a recognized source of error that can affect the final EFW result. In cases of multiple gestation, the EFW for the non-presenting or smaller twin tends to have a slightly higher margin of error.