FL Hadlock on an ultrasound report refers to your baby’s femur length (FL) measured and plotted against growth charts developed by Dr. Frank Hadlock, a radiologist whose formulas became the most widely used standard for estimating fetal size and weight. If you see this on your scan results, it means the sonographer measured the length of your baby’s thighbone and used Hadlock’s reference data to determine whether that measurement falls within a normal range for your baby’s gestational age.
What Femur Length Measures
Femur length is the measurement of your baby’s thighbone, the longest bone in the body. It’s one of four standard measurements taken during routine prenatal ultrasounds after 14 weeks of pregnancy. The other three are biparietal diameter (the width of the head), head circumference, and abdominal circumference. Together, these four numbers give your provider a detailed picture of how your baby is growing.
The sonographer measures only the hard, calcified shaft of the femur, not the softer cartilage at either end. This is why your report may say “femur diaphysis length.” The bone needs to be clearly visible in the image, ideally lying horizontally on the screen, so the calipers can be placed precisely at each end of the shaft. Small errors in placement can shift the measurement by a few millimeters, which matters when millimeters define the difference between one percentile and another.
How Hadlock’s Formulas Work
Frank Hadlock published a series of regression formulas in the 1980s and 1990s that convert ultrasound measurements into an estimated fetal weight. His most widely used formula combines three parameters: abdominal circumference, head circumference, and femur length. Both the National Institute of Child Health and Human Development (NICHD) and the World Health Organization built their fetal weight growth standards on this formula.
The idea is straightforward. No single measurement captures overall fetal size well enough on its own. A baby’s head can be large while the abdomen is small, or vice versa. By combining measurements from different body parts, the formula accounts for normal variation in body proportions and produces a more reliable weight estimate. Hadlock’s three-parameter formula has consistently performed well across both low-risk and high-risk pregnancies, which is why it became the default in many ultrasound machines worldwide.
Typical Femur Length by Gestational Age
Femur length grows rapidly during pregnancy, roughly tripling between 14 and 20 weeks, then growing more steadily through the third trimester. Here are representative values from NICHD reference data showing the 5th, 50th, and 95th percentiles in millimeters:
- 14 weeks: 9 mm (5th) to 11 mm (50th) to 13 mm (95th)
- 20 weeks: 27 mm (5th) to 32 mm (50th) to 38 mm (95th)
- 27 weeks: 45 mm (5th) to 50 mm (50th) to 56 mm (95th)
- 32 weeks: 68 mm (5th) to 73 mm (50th) to 80 mm (95th)
- 37 weeks: 73 mm (5th) to 78 mm (50th) to 84 mm (95th)
These ranges are broad, and most healthy babies fall somewhere between the 5th and 95th percentiles. A measurement at the 20th percentile doesn’t indicate a problem. It means your baby’s femur is on the shorter side of normal, which can simply reflect your family’s stature and genetics.
What a Short Femur Length Can Mean
A femur length below the 5th percentile gets more attention. In most cases, it turns out to be a normal variant, especially when the parents are shorter than average. Constitutionally small fetuses often have shorter femurs without any underlying problem.
Rarely, a very short femur can be associated with skeletal dysplasias (conditions affecting bone growth) or chromosomal differences such as trisomy 21. This is why a measurement below the 5th percentile often triggers a closer look. Your provider may check whether the other measurements (head, abdomen) are proportional, review the anatomy scan for any other findings, and consider your overall risk profile. A short femur in isolation, with everything else looking normal, is far less concerning than a short femur combined with other unusual findings.
How Accurate the Estimates Are
No ultrasound formula predicts actual birth weight perfectly. The Hadlock formula’s weight estimates are generally within about 10 to 15 percent of the true birth weight. In studies comparing its performance to other formulas, Hadlock consistently shows the strongest correlation between predicted and actual weight, with a coefficient of determination (a measure of how well the prediction matches reality) of 0.77, outperforming newer alternatives.
Accuracy drops in certain situations. Babies who are very small or very large for gestational age are harder to measure precisely. Positioning also matters: if the baby is curled tightly or the femur isn’t well visualized, the measurement may be less reliable. This is one reason repeat scans are common when a measurement looks off.
Hadlock Versus Newer Growth Standards
While Hadlock remains the most commonly programmed formula in ultrasound machines, it isn’t the only option. Newer standards from the WHO and the INTERGROWTH-21st project offer some advantages, particularly the ability to track growth velocity (how quickly a baby is gaining weight between scans, not just how big it is at one moment). Hadlock’s original data was based on cross-sectional ultrasounds, meaning each baby was measured once rather than tracked over time, which limits its usefulness for detecting slowing growth.
A 2024 comparison in a U.S. population found that all the major standards performed similarly when predicting serious complications. The practical difference is where they draw the line: Hadlock tends to classify fewer babies as small for gestational age compared to WHO and NICHD standards, and it classified only 8.3% of babies as large for gestational age near term, closer to the expected 10% than other charts. This matters because crossing a threshold on one chart but not another can change how closely your pregnancy is monitored. There is currently no consensus in the U.S. on which chart should be used, which is why you may see different classifications depending on the facility.
Reading Your Ultrasound Report
When you see “FL Hadlock” on your report, look for two pieces of information: the raw measurement in millimeters and the percentile. The millimeter number tells you the actual length of the femur. The percentile tells you where that measurement falls compared to other babies at the same gestational age using Hadlock’s reference data. A value at the 50th percentile means your baby’s femur is right at the median. A value at the 25th percentile means 25% of babies have a shorter femur at that age, which is completely normal.
Your report may also include an estimated fetal weight that was calculated using the Hadlock formula. This number combines the femur length with your baby’s head and abdomen measurements. It’s listed in grams and sometimes converted to pounds. Keep in mind this is an estimate with a margin of error, not a precise weight. Providers use it to track a trend over multiple visits rather than treating any single number as definitive.