FL and BPD are two standard measurements taken during a prenatal ultrasound. FL stands for femur length, the length of your baby’s thighbone. BPD stands for biparietal diameter, the width of your baby’s skull measured from one side to the other. Together, these measurements help estimate gestational age, track fetal growth, and flag potential concerns.
What Each Measurement Tells You
BPD measures the distance in millimeters between the two parietal bones, the broad, flat bones on either side of your baby’s head. To get an accurate reading, the sonographer captures a specific cross-section of the skull where the thalami (a structure deep in the brain) and the cavum septum pellucidum (a small fluid-filled space near the front of the brain) are both visible. This standardized view ensures the measurement is taken at the widest, most reproducible part of the head.
FL measures the length of the femur, the longest bone in the body, also in millimeters. Because bone shows up clearly on ultrasound, the femur is one of the easiest and most reliable structures to measure. It gives a snapshot of skeletal growth that complements what head measurements reveal about brain and skull development.
You’ll usually see FL and BPD reported alongside two other standard measurements: HC (head circumference) and AC (abdominal circumference). These four numbers together form the core of fetal biometry, the set of measurements used to estimate your baby’s size and weight at any point in pregnancy.
How FL and BPD Estimate Gestational Age
Both measurements are used to calculate how far along a pregnancy is, but their accuracy depends on timing. Between 12 and 26 weeks, BPD can estimate gestational age to within about 10 to 11 days. FL during that same window is slightly less precise, accurate to within 10 to 20 days. After 26 weeks, both measurements lose accuracy, and by the third trimester each carries an error margin of roughly two to three weeks.
This is why early ultrasounds are so valuable for dating. A first-trimester scan using crown-rump length (the full length of the embryo) is accurate to within about five days, and guidelines recommend against redating a pregnancy based on later scans. If a woman has her first ultrasound in the third trimester, clinicians typically estimate gestational age using head circumference plus femur length, though even the best method at 32 weeks still has about a 15-day margin of variation.
Normal Growth Rates by Trimester
Both the femur and the skull grow fastest in the second trimester, then gradually slow down. Data from the INTERGROWTH-21st Project, a large international study, shows how these growth rates shift week by week.
Femur length grows at about 3.2 mm per week at 16 weeks, slowing to 2.2 mm per week at 28 weeks and 1.4 mm per week by 38 weeks. The decline is roughly linear as pregnancy progresses. BPD follows a similar pattern but peaks slightly later: it holds steady at about 3.2 mm per week from 16 to 20 weeks, then gradually drops to 2.3 mm per week at 32 weeks and just 1.0 mm per week near term.
Your care provider plots these measurements on standardized growth charts to see where your baby falls relative to other babies at the same gestational age. A single measurement that’s a little above or below average is rarely concerning on its own. What matters more is the pattern over time: consistent growth along a percentile curve is reassuring, while a sudden drop-off may prompt closer monitoring.
What a Short Femur Length Can Mean
A femur length below the 5th percentile is considered short. This finding is common enough, and it has a wide range of explanations. In many cases, it’s simply a normal variant, particularly when parents are shorter or from certain ethnic backgrounds where average femur length runs smaller.
That said, a short femur can sometimes point to other things. It’s considered a soft marker for trisomy 21 (Down syndrome), meaning it slightly raises the statistical likelihood but is far from diagnostic on its own. It can also be an early sign of intrauterine growth restriction, where the baby isn’t growing as expected due to placental or other issues. In rare cases, a significantly short femur relative to head size may suggest a skeletal dysplasia, a condition affecting bone development.
When a short femur is found, providers typically do a more detailed anatomy scan. If other structural differences are also present, genetic testing is usually offered. If the short femur appears to be part of a pattern of overall smaller size, follow-up ultrasounds every two weeks with blood flow (Doppler) assessments can help determine whether the baby is simply small or genuinely growth-restricted.
What a Large BPD Can Mean
A BPD on the higher end of the range often simply reflects a bigger baby. But because BPD directly measures head width, it has practical implications for delivery. Research has found that higher BPD values are independently associated with a greater likelihood of assisted delivery. Babies with a BPD of 97 mm or above (roughly the top 25% near term) had significantly lower rates of spontaneous vaginal delivery compared to those with smaller head measurements. Importantly, though, the larger head size didn’t appear to cause worse outcomes for the baby itself; it mainly affected how the delivery unfolded.
BPD can also appear artificially high or low depending on head shape. Some babies have a naturally rounder head, others a more elongated one. This variation is captured by the cephalic index, which is the ratio of head width to head length. When the head is unusually round, BPD can overestimate head size, and when it’s elongated, BPD can underestimate it. This is one reason head circumference is often used alongside or instead of BPD in the third trimester, since circumference isn’t affected by head shape. Measuring BPD also becomes harder late in pregnancy when the baby’s head is deep in the pelvis.
The FL/BPD Ratio
Sometimes you’ll see FL and BPD expressed as a ratio rather than individual numbers. The BPD-to-FL ratio changes predictably as pregnancy progresses because the head and femur grow at different rates. Early in pregnancy, the head is proportionally much larger than the thighbone, so the ratio starts high (above 3 before 13 weeks) and drops below 2 by around 18 weeks as limb growth catches up.
This ratio is especially useful for screening rare but serious skeletal conditions. In thanatophoric dysplasia, a lethal skeletal disorder, the limbs are extremely short relative to the head. A BPD/FL ratio that stays abnormally high for gestational age can raise a red flag for this condition as early as the first trimester. For the vast majority of pregnancies, though, the ratio falls well within the expected range and simply confirms that head and limb growth are proportional to each other.
Reading Your Ultrasound Report
When you look at your ultrasound report, you’ll typically see FL and BPD listed in millimeters alongside a corresponding gestational age estimate for each measurement. It’s normal for these individual age estimates to differ by a week or so. Your baby’s head might measure “on track” for 24 weeks while the femur reads closer to 23 weeks, and that discrepancy alone doesn’t indicate a problem.
You may also see percentile rankings. These tell you where your baby’s measurement falls compared to a reference population. A femur at the 30th percentile means 30% of babies at that gestational age have a shorter femur and 70% have a longer one. Anything between roughly the 10th and 90th percentile is generally considered within the normal range, though the specific cutoffs your provider uses may vary slightly.