Growth ultrasounds provide an estimate of the baby’s size and growth rate, known as the Estimated Fetal Weight (EFW). Healthcare providers use the EFW to monitor the baby’s progression and identify deviations from typical growth patterns. Understanding the accuracy of this projection is important for expectant parents and informs clinical decisions regarding pregnancy and delivery. Since the EFW is a calculation based on geometric assumptions rather than a direct measurement of mass, it carries uncertainty.
How Fetal Weight is Estimated
To calculate the Estimated Fetal Weight, the sonographer uses ultrasound technology to take several standardized measurements of the fetus’s anatomy, known as fetal biometrics. These measurements are obtained using sound waves that reflect off the baby’s tissues. Four primary measurements are typically taken: the Biparietal Diameter (BPD) and Head Circumference (HC) for the head, the Abdominal Circumference (AC) for the trunk, and the Femur Length (FL) for the long bones.
The Abdominal Circumference is the most significant measurement because it reflects the size of the fetal liver and subcutaneous fat, which are the primary determinants of weight gain. These four data points are entered into established mathematical formulas, such as those developed by Hadlock. These formulas use statistical modeling to convert the linear measurements into a volumetric weight estimation, which is then compared to growth curves to determine the baby’s growth percentile.
The Typical Margin of Error
The Estimated Fetal Weight is generally considered accurate within a range of 10% to 15% of the baby’s actual birth weight. This established range means that if an ultrasound predicts a baby weighs 7 pounds (3,175 grams), the actual weight upon delivery could fall anywhere between 6 pounds, 3 ounces and 7 pounds, 11 ounces, assuming a 10% margin of error. The confidence limit for the most commonly used formulas, such as the Hadlock four-parameter equation, is typically plus or minus 15%, meaning that 95% of babies will be born within that weight range of the estimate.
The uncertainty in the EFW calculation increases significantly as the pregnancy advances, particularly in the late third trimester. As the fetus grows larger and the space within the womb becomes more restricted, obtaining precise, clear measurements of the head and abdomen becomes increasingly challenging. The accuracy is also reduced at the extremes of fetal size, meaning the estimates for very small or very large babies are often less reliable than for those of average weight.
There is a systematic tendency for the ultrasound calculation to err in a specific direction when dealing with these size extremes. The EFW often overestimates the weight of fetuses that are truly small for gestational age, which could lead to a less urgent clinical response. Conversely, the calculation frequently underestimates the weight of fetuses that are large for gestational age or macrosomic.
For example, a baby estimated to weigh 9 pounds might actually be 10 pounds or more, which carries implications for delivery planning. This phenomenon is partly due to the fact that the formulas rely on averages and may not fully account for the disproportionate growth of body parts in fetuses at the very top or bottom of the growth curve. Consequently, clinicians must interpret the EFW not as a precise scale reading but as an approximation with a considerable range of potential actual outcomes.
Variables That Affect Accuracy
Several physical and technical variables introduce uncertainty beyond the mathematical limitations of the EFW formulas. The position of the fetus inside the uterus greatly affects the ease and accuracy of obtaining biometric data. If the baby is oriented in a way that obscures the head or abdomen, the sonographer may have difficulty getting a clear image. This can lead to imprecise caliper placement and erroneous measurements.
Maternal body characteristics also play a role. Increased maternal body fat can scatter ultrasound waves, resulting in lower-quality images that make fetal structures harder to delineate. Similarly, the volume of amniotic fluid surrounding the baby impacts image clarity and the distance the sound waves must travel.
The expertise and experience of the sonographer performing the scan significantly influences the quality of the raw data. Variability in how different operators place the electronic calipers introduces errors before the data is entered into the calculation formula. Furthermore, the time elapsed between the ultrasound and delivery diminishes the EFW’s reliability, as the baby continues to grow during that interval.
What the Results Mean for Delivery Planning
Despite the known margin of error, the Estimated Fetal Weight remains valuable information guiding obstetric management decisions. When the EFW suggests a baby is very large, known as suspected macrosomia, it prompts discussions about potential risks during vaginal delivery, such as shoulder dystocia. This estimate may lead a provider to recommend an elective induction of labor or a planned cesarean delivery. However, the possibility of overestimation means some interventions may be unnecessary.
Conversely, an EFW below the 10th percentile suggests Fetal Growth Restriction (FGR), signaling potential problems with the baby’s health or placental function. In these cases, the EFW justifies increased fetal surveillance, including frequent monitoring using Doppler studies and biophysical profiles. The EFW is never used in isolation; it is always assessed alongside other factors like the mother’s health history and fundal height measurements. When combined with these clinical markers, the EFW may influence the decision to pursue an early delivery to mitigate risks associated with remaining in the womb.