Estimated Fetal Weight (EFW) provides a non-invasive assessment of a baby’s size during pregnancy. This measurement is particularly relevant at 34 weeks, as the fetus is rapidly growing and approaching the final stages of gestation. EFW is not a precise scale reading but rather a calculated estimate derived from various two-dimensional measurements. The primary purpose of this estimate is to monitor the growth trajectory of the fetus, ensuring it aligns with expected developmental milestones.
How Estimated Fetal Weight is Calculated
EFW is generated by a sonographer taking specific measurements of the fetus’s physical structures, known as fetal biometry. These measurements include the Biparietal Diameter (BPD), Head Circumference (HC), and Femur Length (FL). The most influential measurement is the Abdominal Circumference (AC), which indicates the fetus’s soft tissue bulk.
These four distinct measurements are input into a mathematical formula, most commonly one of the Hadlock formulas, which uses regression analysis to generate the final weight estimate in grams. The formula transforms the linear measurements into a volumetric estimate of mass. Because the calculation uses multiple body parts, it is a more reliable indicator of fetal size than any single measurement alone. The resulting EFW is then plotted on a growth chart to determine the percentile ranking for that gestational age.
Understanding the Accuracy Range at 34 Weeks
The accuracy of an EFW reading is generally reliable but always includes a margin of error. In the third trimester, including at 34 weeks, the error margin for the EFW is typically between +/- 10 and +/- 15 percent of the baby’s actual birth weight. This means a baby estimated by ultrasound to weigh 5 pounds could realistically weigh anywhere from 4 pounds, 4 ounces to 5 pounds, 12 ounces.
Accuracy tends to decrease as the pregnancy progresses because the fetus grows larger and becomes more compressed within the uterus. This crowding makes it increasingly difficult for the sonographer to obtain clear, precise cross-sectional images, especially of the Abdominal Circumference. The mathematical models can also struggle with extremes, often overestimating the weight of very small babies and underestimating the weight of very large babies.
Biological and technical factors introduce variability into the final EFW number. A high maternal body mass index (BMI) or low amniotic fluid levels can create suboptimal imaging conditions, making it harder to define the fetal boundaries accurately. The skill and experience of the sonographer also play a role, as the slightest deviation in caliper placement can compound the error when the measurements are run through the regression equation.
What Estimated Fetal Weight Means for Delivery Planning
Despite the +/- 15 percent margin of error, EFW remains important clinical data for monitoring fetal well-being and planning delivery. At 34 weeks, EFW is used to categorize growth against standard curves. A fetus below the 10th percentile is classified as Small for Gestational Age (SGA), while one above the 90th percentile is considered Large for Gestational Age (LGA) or suspected macrosomia.
These classifications prompt increased monitoring due to the associated risks of perinatal morbidity and mortality. For an SGA diagnosis, the care team may recommend more frequent growth scans or non-stress tests to check on the baby’s health and placental function. Conversely, a suspected LGA diagnosis alerts the team to potential delivery complications such as shoulder dystocia, where the baby’s shoulder gets stuck after the head is delivered.
While EFW alone rarely dictates the timing of delivery, it is a significant factor when combined with other health indicators. In cases of suspected macrosomia, a high EFW may lead to a discussion about a planned cesarean section to avoid injury to the baby or mother. For suspected growth restriction, a concerning EFW trend may contribute to the decision to proceed with an earlier delivery or induction, especially if other tests suggest the baby is not thriving in the uterine environment.