EFW stands for estimated fetal weight, and it’s the number on your ultrasound report that tells you roughly how much your baby weighs while still in the womb. It’s calculated by plugging several measurements of your baby’s body into a mathematical formula, and it’s one of the main tools your provider uses to track whether your baby is growing on schedule. The estimate carries a margin of error of about 10 to 15%, which means it’s a useful guide rather than an exact number.
How EFW Is Measured
During an ultrasound, the sonographer takes a series of measurements of your baby’s anatomy. The key ones are the diameter across the head, the circumference around the head, the circumference around the belly, and the length of the thigh bone. In the first trimester, a simpler top-of-head-to-bottom measurement is more reliable, but after about 12 weeks those four measurements become the standard way to assess growth.
The ultrasound machine runs these numbers through a built-in formula to generate the weight estimate. There are actually more than 20 different formulas programmed into modern ultrasound systems, most developed in the 1980s. The most widely used ones come from the Hadlock group of researchers. The version that combines head circumference, belly circumference, and thigh bone length tends to produce the lowest error rates, with a standard deviation of about 7.6% from the actual birth weight. Adding additional head measurements barely improves accuracy beyond that.
What the Percentile Number Means
Your EFW is usually reported alongside a percentile, which compares your baby’s estimated weight to other babies at the same stage of pregnancy. A baby at the 50th percentile is right in the middle of the range. The zone between the 10th and 90th percentiles is generally considered appropriate for gestational age.
If the EFW falls below the 10th percentile, your baby may be classified as “small for gestational age” (SGA). This doesn’t automatically mean something is wrong. Some babies are simply small because their parents are small. The important distinction is between a baby who is small but healthy and one whose growth has been restricted by a problem like poor blood flow through the placenta. Restricted growth is typically identified by a pattern of slowing growth over time, abnormal blood flow on Doppler ultrasound, or other signs of placental trouble, not just a single low weight reading.
On the other end, a weight above the 90th percentile is called “large for gestational age” (LGA). Specific weight thresholds also come into play near your due date. A baby estimated at over 4,000 grams (about 8 pounds 13 ounces) is often described as macrosomic. Complication risks, particularly shoulder dystocia during vaginal delivery, rise modestly at that weight and climb more sharply above 4,500 grams (9 pounds 15 ounces).
When EFW Is Typically Checked
Most pregnancies include an anatomy scan around 18 to 22 weeks, which gives an early weight estimate. The most common timing for a dedicated growth scan is 32 to 34 weeks, when the baby is large enough to get a meaningful weight reading but there’s still time to act if a growth problem is identified. Higher-risk pregnancies, such as those involving diabetes, high blood pressure, or a history of growth problems, may have growth scans earlier or more frequently.
The closer the scan is to your actual delivery date, the more useful the weight estimate is for planning delivery. But there’s a tradeoff: the further along you are, the harder it can be to get clear images, and the margin of error tends to increase as the baby gets bigger.
How Accurate the Estimate Really Is
EFW is an estimate, and it’s worth understanding what that means in practical terms. Using the best available formulas, the true weight will fall within about 15% of the estimate 95% of the time. For a baby estimated at 3,500 grams (7 pounds 11 ounces), that means the actual weight could reasonably be anywhere from about 2,975 to 4,025 grams, a range of more than two pounds.
Several factors can push accuracy in one direction or the other. Higher maternal BMI makes it harder for ultrasound waves to produce clear images, which reduces measurement precision. Older maternal age is also associated with less accurate readings. A longer gap between the ultrasound and delivery gives the baby more time to grow beyond what was estimated, making the number less reflective of actual birth weight. One study found that the interval between the scan and delivery was the single strongest predictor of an inaccurate estimate, with the odds of a significant error jumping more than sevenfold when that gap was longer.
Larger babies are also harder to measure accurately. This is particularly relevant when providers are trying to predict macrosomia, since the estimates are least reliable at the high end of the weight range, exactly where precision matters most for delivery planning.
What Happens if EFW Is Too Low or Too High
A single EFW that falls outside the normal range doesn’t necessarily change your care plan on its own. What matters more is the trend. If your baby measured at the 20th percentile at 28 weeks and drops to the 5th percentile at 34 weeks, that falling trajectory is more concerning than a baby who has consistently tracked along the 8th percentile from the start.
For babies measuring small, your provider will typically order follow-up scans to watch the growth pattern, along with Doppler ultrasound to check blood flow through the umbilical cord. If there are signs that the placenta isn’t delivering enough nutrients or oxygen, earlier delivery may be recommended.
For babies measuring large, the main concern is whether vaginal delivery can proceed safely. The risk of shoulder dystocia, where the baby’s shoulder gets caught behind the pelvic bone during delivery, rises from about 1% for babies under 4,000 grams to 5 to 10% for babies between 4,000 and 4,500 grams. Planned cesarean delivery is generally considered when the estimated weight exceeds 5,000 grams, or 4,500 grams in pregnancies complicated by gestational diabetes.
In both cases, the EFW is one piece of information your provider weighs alongside your medical history, physical exam, and other test results. Because the estimate carries a meaningful margin of error, it rarely drives major decisions on its own.