An Estimated Fetal Weight (EFW) measurement via ultrasound at 37 weeks is often a source of both curiosity and concern for expectant parents. Since the fetus is considered near-term, the weight estimate can influence discussions about labor, delivery planning, and potential risks. Understanding the reliability of this number is important for setting appropriate expectations, as the ultrasound provides an estimate rather than a precise measurement. The EFW is a calculation that allows healthcare providers to assess if the baby is growing as expected relative to others at the same gestational age.
Measuring Fetal Weight: The Calculation Methods
The Estimated Fetal Weight is derived from several measurements of the baby’s body taken during the ultrasound scan. Sonographers input specific parts of the fetal anatomy into complex mathematical formulas, such as those developed by Hadlock, which rely on biometric data.
These measurements typically include the Head Circumference (HC), the Abdominal Circumference (AC), and the Femur Length (FL). The Abdominal Circumference is the most influential measurement in determining the EFW because it reflects the size of the liver, a primary storage site for fetal glycogen and fat. The formula translates these two-dimensional measurements into a three-dimensional weight estimate in grams.
The Expected Margin of Error at 37 Weeks
The accuracy of the EFW is inherently limited because it relies on measuring a moving, three-dimensional object using two-dimensional technology. At 37 weeks, the estimated weight is expected to fall within a margin of error of \(\pm\) 10% to 15% of the baby’s actual birth weight. This means a baby estimated to weigh 3,000 grams could realistically weigh anywhere from 2,550 grams to 3,450 grams. The margin of error can sometimes exceed 20% of the birth weight, especially for babies at the extremes of size.
The accuracy often decreases in the final weeks of pregnancy due to physical changes within the womb. The fetus is growing rapidly and occupies more space, which leads to increased crowding and limited mobility. This cramped environment makes it difficult to obtain a perfect cross-section for measurements. The Abdominal Circumference, in particular, may be compressed or distorted. The baby’s head often descends deeper into the mother’s pelvis at this stage, a process known as engagement, which can obscure the Head Circumference measurement.
Advanced gestational age also presents technical challenges. Reduced amniotic fluid around the baby lessens the acoustic window for the ultrasound waves. The increased calcification and density of the fetal bones can also create shadows that hinder clear visualization of the soft tissue boundaries. These factors contribute to higher variability and decreased precision of the EFW calculation compared to earlier in the second trimester. The EFW tends to be least accurate for babies who are either very small or very large for their gestational age.
Interpreting EFW Results and Clinical Next Steps
Medical professionals use the EFW not as a definitive number, but as a tool to determine the baby’s growth percentile. This percentile compares the baby’s estimated weight to that of other babies at the same gestational age. It identifies whether the baby is Small for Gestational Age (SGA), typically below the 10th percentile, or Large for Gestational Age (LGA), typically above the 90th percentile. This context is more important than the weight number alone, as it helps determine the risk of growth-related complications.
An EFW suggesting a very large or very small baby at 37 weeks rarely leads to immediate delivery decisions on its own. The estimated weight is always considered alongside a broader clinical picture. This includes reviewing the baby’s growth trend over multiple previous scans, assessing the mother’s health conditions, such as gestational diabetes, and evaluating the volume of amniotic fluid.
Doctors also look at Doppler flow studies, which assess blood flow in the umbilical cord and fetal vessels, providing insight into placental function. A concern about an abnormally large baby, sometimes called macrosomia, or an abnormally small baby requires a comprehensive risk assessment that factors in the known error margin of the EFW. The final decision regarding the timing and method of delivery is based on this totality of evidence, not just the single weight estimate from the ultrasound.