Your baby’s weight on an ultrasound report is listed as the EFW, which stands for Estimated Fetal Weight. It’s usually displayed in grams or pounds near the bottom of the report, after the individual body measurements the machine used to calculate it. If you can’t spot “EFW” on your printout, look for “EFBW” (estimated fetal birth weight) or simply “Wt,” as some machines label it differently.
Understanding what that number means, how accurate it is, and whether it falls in a normal range takes a bit more context. Here’s how to read the key parts of your report.
The Four Measurements Behind the Weight
An ultrasound machine doesn’t weigh your baby directly. It measures parts of the baby’s body, then plugs those numbers into a mathematical formula to estimate weight. The four measurements you’ll see on nearly every second- and third-trimester report are:
- BPD (biparietal diameter): the width of the baby’s head, measured from one side of the skull to the other.
- HC (head circumference): the distance around the baby’s head.
- AC (abdominal circumference): the distance around the baby’s belly, which reflects liver size and fat stores.
- FL (femur length): the length of the thighbone, used as a proxy for overall body length.
The most widely used formula, developed by Hadlock, combines the head circumference, abdominal circumference, and femur length to produce the EFW. Some machines use all four measurements; others use just two or three. The more measurements included, the more accurate the estimate tends to be.
Where to Find EFW on Your Report
Most ultrasound reports list each individual measurement (BPD, HC, AC, FL) in a table or column, often with a corresponding gestational age for that measurement. The EFW typically appears below or beside these values, reported in grams. A report at 34 weeks might read something like “EFW: 2,377 g” or “EFW: 5.2 lbs.”
Next to the weight, you may also see a percentile. This tells you how your baby’s estimated weight compares to other babies at the same gestational age. A 50th percentile means the baby is right at the median. A 25th percentile means the baby is larger than 25% of babies at that stage and smaller than the other 75%. Values below the 3rd percentile or above the 97th typically get flagged on the report.
What the Percentile Tells You
The percentile is often more clinically meaningful than the raw weight in grams. A baby weighing 2,100 grams might sound small, but if the pregnancy is at 33 weeks, that’s close to the average of about 2,162 grams and perfectly normal. The same weight at 36 weeks would fall well below the median of 2,813 grams and raise questions about growth.
Babies with an estimated weight below the 10th percentile for their gestational age are classified as “small for gestational age,” or SGA. This doesn’t automatically mean something is wrong. Some babies are constitutionally small, especially if the parents are smaller. But it can prompt your provider to schedule follow-up scans to track growth over time. Less commonly, cutoffs at the 5th or 3rd percentile are used for stricter definitions.
On the other end, babies above the 90th percentile are considered “large for gestational age,” or LGA. Normal weight at full term ranges from about 2,500 to 4,200 grams (roughly 5.5 to 9.3 pounds).
Average Fetal Weight by Week
If your report gives a weight in grams but no percentile, you can compare it to these averages for the third trimester:
- 28 weeks: 1,210 g (2.7 lbs)
- 30 weeks: 1,559 g (3.4 lbs)
- 32 weeks: 1,953 g (4.3 lbs)
- 34 weeks: 2,377 g (5.2 lbs)
- 36 weeks: 2,813 g (6.2 lbs)
- 38 weeks: 3,236 g (7.1 lbs)
- 40 weeks: 3,619 g (8.0 lbs)
These are median values, so roughly half of all babies will weigh more and half will weigh less at each stage. A baby that’s a few hundred grams above or below these numbers is still well within normal range.
How Accurate the Estimate Really Is
The accepted margin of error for ultrasound weight estimates is plus or minus 15%. That means if your report says the baby weighs 3,000 grams, the actual weight at birth could reasonably fall anywhere between about 2,550 and 3,450 grams. This is the accuracy level for the best-performing Hadlock formula (the one using head circumference, abdominal circumference, and femur length together). Simpler formulas using fewer measurements have wider margins, up to plus or minus 22% when only the abdominal circumference is used.
Several factors can push the estimate further from reality. Higher maternal BMI makes it harder for the ultrasound to get clean images, which affects measurement precision. Amniotic fluid volume matters too: very low or very high fluid levels can distort the landmarks the sonographer is measuring. Fetal position plays a role, as a baby curled tightly or facing a difficult angle makes certain measurements harder to capture cleanly. Even the time gap between the scan and delivery affects how well the estimate matches birth weight, since the baby continues gaining roughly 200 grams per week in the final month.
When Growth Scans Are Typically Done
In low-risk pregnancies, fetal weight is usually estimated during the anatomy scan around 18 to 22 weeks, and your provider may order one additional growth scan in the third trimester if there’s a reason to check. Fundal height, the tape-measure check of your belly at each prenatal visit after 24 weeks, serves as the primary screening tool for growth. An ultrasound is ordered when fundal height doesn’t match expectations or when risk factors are present.
For higher-risk pregnancies, including those complicated by high blood pressure, diabetes, or a previous growth-restricted baby, serial growth scans are common. International guidelines recommend the third-trimester scan between 32 and 36 weeks, with recent large studies pointing to 36 weeks as the best single time point for catching late-onset growth problems in otherwise low-risk pregnancies. For early-onset concerns, scans around 30 to 32 weeks are more useful.
Making Sense of Your Specific Report
Start by locating the EFW line and noting whether it’s reported in grams or pounds. Then look for a percentile. If there’s no percentile listed, compare the weight to the averages for your gestational week using the table above. Check whether the individual measurements (BPD, HC, AC, FL) are all tracking at a similar gestational age. When one measurement lags significantly behind the others, it can signal asymmetric growth, where the baby’s head is growing normally but the belly is smaller, for example. This pattern is more clinically significant than all measurements being uniformly a little small.
Keep in mind that a single scan is a snapshot. Growth trends across two or more scans, typically spaced two to three weeks apart, give a much clearer picture than any one number. A baby that drops from the 40th percentile to the 10th percentile over several weeks is more concerning than a baby that’s consistently been at the 8th percentile throughout pregnancy.