Fetal femur length grows from roughly 1 cm at 14 weeks of gestation to about 7.5 cm near term, and standardized reference charts plot the expected range for each week of pregnancy. These charts, built from large ultrasound studies of healthy pregnancies, give clinicians a way to confirm gestational age, estimate fetal weight, and flag potential growth problems. The values most parents encounter on their ultrasound reports come from reference data published in the 1990s and updated periodically since, and understanding what those numbers mean in practice involves more than just finding your week on a table.
Approximate Femur Length Values by Week
The most widely used reference charts come from cross-sectional studies that measured each fetus only once, then built statistical models to estimate the average and normal spread at every gestational week. One influential chart, published in the British Journal of Obstetrics and Gynaecology, used measurements from 649 fetuses scanned between 12 and 42 weeks and accounted for the fact that the normal range widens as pregnancy progresses.1PubMed. Charts of fetal size: 4. Femur length The approximate 50th-percentile values in centimeters, rounded to one decimal, look something like this:
- 14 weeks: 1.5 cm
- 16 weeks: 2.1 cm
- 18 weeks: 2.7 cm
- 20 weeks: 3.3 cm
- 22 weeks: 3.8 cm
- 24 weeks: 4.4 cm
- 26 weeks: 4.9 cm
- 28 weeks: 5.4 cm
- 30 weeks: 5.8 cm
- 32 weeks: 6.2 cm
- 34 weeks: 6.6 cm
- 36 weeks: 6.9 cm
- 38 weeks: 7.2 cm
- 40 weeks: 7.5 cm
These are median values. A healthy fetus can fall anywhere between roughly the 5th and 95th percentiles and still be considered within normal range. At 20 weeks, for instance, the 5th percentile might be about 2.9 cm and the 95th percentile about 3.7 cm. The spread increases as pregnancy advances, so by 36 weeks the gap between the 5th and 95th percentiles can be over a centimeter. That widening variability is not a flaw in the chart; it reflects real biological differences in how babies grow.1PubMed. Charts of fetal size: 4. Femur length
Growth is also not perfectly linear. Early in the second trimester, the femur elongates quickly, adding several millimeters per week. After about 30 weeks the rate of growth slows, which is why the weekly increments on the chart get smaller as you approach term. Early studies of fetal limb bones noted that growth appeared linear from 12 through 22 weeks but then curved, and the femur was the first bone to become clearly visible on ultrasound and the easiest to measure reproducibly.
How the Measurement Is Taken
Femur length is measured on a standard prenatal ultrasound by aligning the probe so the full length of the thighbone appears in a single image. The sonographer looks for both ends of the bone, where the bright echoes of the shaft taper into the cartilage caps at each end. Only the calcified shaft is measured, from one end to the other, excluding the softer cartilage that has not yet turned to bone.
Getting this right is trickier than it sounds. If the ultrasound beam catches the femur at an angle rather than straight on, the bone looks shorter than it actually is. On the other hand, if the sonographer accidentally includes part of the uncalcified cartilage at either end, the measurement comes out too long. A classic study on femur-length pitfalls found that both errors can be avoided by capturing a view that shows the cartilage caps on each side, which confirms the entire shaft is in the image and makes the boundaries of the bony portion clear.2PubMed. Pitfalls in femur length measurements
Fetal position matters too. A baby whose legs are tucked tightly or who is facing the mother’s spine can make it harder to get that ideal straight-on view. This is one reason your sonographer may spend time nudging you to shift position or may ask you to come back for a repeat scan.
How Much Measurement Variation Is Normal
Even skilled sonographers do not get the exact same number every time they measure the same femur. Studies that have quantified this show that femur length has more measurement-to-measurement variability than head circumference. In one large analysis, the 95% limits of agreement for the same sonographer re-measuring the femur were about plus or minus 6.6%, and when two different sonographers measured the same fetus, the spread widened to about plus or minus 11%.3PubMed. Intra- and interobserver variability in fetal ultrasound measurements Data from the INTERGROWTH-21st project, a large international quality-control effort, found similar figures, with same-observer limits of agreement around plus or minus 6.2% and between-observer limits around plus or minus 5.6% for femur length.4PubMed. Quality control of ultrasound for fetal biometry: results from the INTERGROWTH-21st Project
What this means in practice: at 30 weeks, when the average femur is about 5.8 cm, a 6% variation amounts to roughly 3.5 mm. That is enough to shift a measurement from the 50th percentile to the 30th or the 70th on a single scan. One reading that looks slightly short is not cause for alarm by itself, which is why clinicians look at trends over multiple visits rather than reacting to a single number.
Why Clinicians Track Femur Length
Estimating Gestational Age
When the date of conception is uncertain, or when the head is in a position that makes it hard to measure, femur length can serve as a backup for estimating how far along a pregnancy is. Early research showed that femur length was about as accurate as head diameter for dating purposes in the second trimester.5PubMed. Assessment of gestational age in the second trimester by real-time ultrasound measurement of the femur length Combining multiple long-bone measurements can improve estimates further when the head measurement alone is unreliable.6PubMed. Estimation of gestational age from measurements of fetal long bones Later in pregnancy, dating becomes less precise with any single measurement. One study found that after 24 weeks, femur length could predict gestational age to within about 11 days, which was slightly less accurate than kidney length but better than abdominal circumference.7PubMed. Determination of gestational age after the 24th week of gestation from fetal kidney length measurements
Estimating Fetal Weight
Femur length is one of the inputs in the Hadlock formulas, the most commonly used equations for estimating fetal weight from ultrasound. The original study found that combining head, abdomen, and femur measurements gave weight estimates with a standard deviation of about 7.5%, which was meaningfully better than formulas that left femur length out.8American Journal of Obstetrics and Gynecology. Estimation of fetal weight with the use of head, body, and femur measurements—A prospective study Including the femur helps because it captures something about skeletal frame size that head and belly measurements alone do not.
There is a caveat, though. When a baby’s growth is asymmetric, meaning the head is growing normally but the belly and limbs are falling behind, including femur length in the weight formula can lead to underestimating the baby’s actual weight. One study found that in asymmetrically growth-restricted preterm babies, formulas using femur length underestimated birth weight by roughly 11 to 13%, while a simpler formula using only head diameter and abdominal circumference came closer to the real weight.9PubMed. Incorporation of femur length leads to underestimation of fetal weight in asymmetric preterm growth restriction This is the kind of clinical nuance that a chart alone cannot capture.
What a Short Femur Length Can Mean
A femur measurement that falls below the 10th or 5th percentile is flagged as “short,” but the cause ranges from completely harmless to clinically serious. The most common explanation is simply that the baby is constitutionally small, reflecting the parents’ own stature and ethnic background. A study of 1,373 fetuses with short femurs during second-trimester screening found that the finding can point toward intrauterine growth restriction, a genetically small-for-gestational-age baby, associated malformations, or adverse outcomes, but for many of these pregnancies the baby is just on the shorter end of normal.10PubMed Central. The femur too short? 1373 fetuses with short femur during second-trimester screening
That said, an isolated short femur below the 10th percentile at mid-pregnancy is associated with a higher chance of fetal growth restriction developing later on. One study found that these pregnancies were about three times more likely to develop growth restriction compared to those with a normal femur length, even after accounting for factors like high blood pressure and tobacco use.11PubMed Central. Isolated Short Femur Length on Mid-Trimester Ultrasound: A Marker for Fetal Growth Restriction and Other Adverse Perinatal Outcomes There is also evidence linking a short femur to placental problems, including preeclampsia. Research has suggested that between 32 and 37 weeks, a short femur can be an early sign of placental insufficiency, potentially preceding other clinical indicators.12Journal of Obstetrics, Gynecology and Cancer Research. Evaluation of the Relationship Between Short Femur Length and Preeclampsia
A short femur is also one of the “soft markers” that can appear on a second-trimester anatomy scan in pregnancies affected by Down syndrome. In isolation, though, it is a weak marker. One study found that a shortened femur carried a likelihood ratio of only 1.5, meaning it barely nudged the risk estimate upward.13PubMed. Isolated sonographic markers for detection of fetal Down syndrome in the second trimester of pregnancy When both the femur and the humerus (upper arm bone) are short, the association is stronger. A study of 45 fetuses with Down syndrome found that those with both short humerus and short femur had an 11-fold greater risk of the condition compared to karyotypically normal fetuses.14American Journal of Obstetrics and Gynecology. Humerus and femur length shortening in the detection of Down’s syndrome Today, with cell-free DNA screening widely available, an isolated short femur alone rarely changes clinical management for chromosome screening.
Skeletal Dysplasias and Extremely Short Femurs
When the femur is severely short, especially below the 1st percentile, clinicians start thinking about skeletal dysplasias, a group of conditions that affect bone and cartilage development. The ratio of femur length to abdominal circumference can help distinguish these from other causes. A study of fetuses with suspected skeletal dysplasias found that all 12 with lethal forms had a femur-length-to-abdominal-circumference ratio below 0.16, while fetuses who turned out to be unaffected all had ratios above 0.18.15PubMed. Ultrasonographic prediction of fetal outcome in suspected skeletal dysplasias with use of the femur length-to-abdominal circumference ratio
More recently, researchers have explored whether these ratios can be used even in the first trimester. A study comparing fetuses with skeletal dysplasias to controls found that the ratio of femur length to biparietal diameter (head width) and the ratio of femur length to foot length were the most useful early screening tools, with the probability of a skeletal dysplasia diagnosis increasing at least ninefold when the femur-to-head ratio fell below a certain threshold.16PubMed. Evaluation of first trimester ultrasound fetal biometry ratios femur length/biparietal diameter, femur length/abdominal circumference and femur length/foot for the screening of skeletal dysplasia For cases where the femur is severely short and disproportionate to the head and abdomen, genomic testing is considered, which can include chromosomal microarray and rapid prenatal exome sequencing.
When the Femur Measures Long
Femur length above the 90th percentile gets less clinical attention than a short measurement, but it is not entirely meaningless. A long femur relative to abdominal circumference usually just means a tall, lean baby. However, when the femur is long but the belly is even bigger, the combination can suggest macrosomia, a baby who is large for gestational age. One study found that macrosomic fetuses had a significantly lower femur-length-to-abdominal-circumference ratio than normal-weight fetuses, and the ratio was particularly useful for identifying macrosomia in pregnancies complicated by maternal diabetes, correctly flagging 89% of that group.17PubMed. Use of femur length/abdominal circumference ratio in detecting the macrosomic fetus
This is a good example of why individual numbers on a chart do not tell the whole story. The clinical picture depends on how femur length relates to other measurements, the gestational age, and the mother’s health status.
What Affects Femur Length Beyond Gestational Age
Parental Height and Ethnicity
The most obvious influence on femur length is the parents’ own body proportions. Taller parents tend to have babies with longer femurs, though the relationship is not as strong in early pregnancy as it is closer to birth. Research from the Pune Maternal Nutrition Study found that neither maternal nor paternal height was associated with femur length at 17 weeks, but by 29 weeks paternal height showed a modest positive relationship.18PubMed Central. Maternal and paternal height and BMI and patterns of fetal growth: The Pune Maternal Nutrition Study The influence of parental stature seems to kick in gradually over the second and third trimesters rather than being present from the start.
Ethnic background matters enough that some clinicians adjust their expectations. A study comparing fetuses across different ethnic groups found significant differences in femur length between populations, along with differences in head and abdominal measurements.19PubMed. Fetal biometry in different ethnic groups Whether to use population-specific charts or a universal standard remains debated. The INTERGROWTH-21st project took the position that under optimal health conditions, fetal growth is similar across populations, while others argue that genetic differences in limb proportions are real and should be reflected in the reference data. In practice, a baby of East Asian descent with a femur at the 8th percentile on a chart built from European data may not warrant the same concern as a baby of Northern European descent at the same percentile.
Maternal Diabetes
Pre-existing diabetes in the mother tends to accelerate fetal growth across all measurements, and the femur is no exception. A study of fetal growth patterns in pregnancies with pregestational diabetes found that all growth rates were higher than in low-risk pregnancies, with the femur showing a z-score of 0.59 above the reference mean.20PubMed. Fetal growth patterns in fetuses of women with pregestational diabetes mellitus The abdominal measurements were even more elevated, reflecting the excess fat and liver glycogen deposition that diabetic macrosomia is known for, but the femur effect is real and can place the measurement above the expected range on a standard chart.
IVF and Twin Pregnancies
Babies conceived through in vitro fertilization may show slightly shorter femur lengths during pregnancy. One study found that IVF fetuses had femur lengths that were about 4 to 7% shorter than spontaneously conceived controls at 24, 30, and 36 weeks, though the difference narrowed somewhat toward term.21PubMed. Is fetal growth impaired after in vitro fertilization? Whether this reflects something about the IVF process itself or about the underlying fertility conditions is unclear.
Twin pregnancies also follow a different growth trajectory. Growth curves for twins diverge from singleton charts, especially during the third trimester when space and placental resources become more limited. The differences are more pronounced in monochorionic twins, who share a placenta, compared to dichorionic twins, who each have their own. Standard femur-length charts built from singleton pregnancies should not be applied directly to twins without this context in mind.
Fetal Sex
Male and female fetuses do grow at slightly different rates, and sex-specific growth curves for femur length have been constructed.22PubMed Central. Sex differences in fetal growth and immediate birth outcomes in a low-risk Caucasian population In practice, the difference in femur length between boys and girls is small enough that most clinical charts do not separate by sex. If you are comparing your baby’s measurement to a chart, this minor difference is unlikely to change the clinical interpretation.
Vitamin D and Femoral Shape
One area of growing interest is whether maternal vitamin D status affects fetal bone development. Low maternal vitamin D has been associated with impaired fetal growth and delayed bone ossification in severe cases. Interestingly, research has found that low vitamin D levels do not seem to shorten the femur itself but may change its shape. At 19 weeks of gestation, low maternal vitamin D was linked to a higher “splaying index” of the fetal femur, a widening of the bone ends that resembles what is seen in childhood rickets. By 34 weeks, maternal vitamin D correlated with femur volume and the diameter of the growth plate at the top of the bone, but not with overall length.23Journal of Bone Metabolism. Gestational Diabetes Mellitus, Fetal Growth and Vitamin D So a mother’s vitamin D level is unlikely to explain why a femur measurement appears short on a chart, but it may affect bone quality in ways that a simple length measurement does not capture.
MRI Studies of the Fetal Femur
Ultrasound measures only the calcified shaft of the femur, but the actual bone includes soft cartilage at each end that will not fully harden until well after birth. Prenatal MRI can visualize more of this structure, and researchers have begun using it to study how the femur develops in growth-restricted pregnancies. MRI studies have found significant differences in femur shape between growth-restricted fetuses and normally growing controls, with bone shortening evident in growth restriction. One finding that stood out was that the secondary ossification center in the lower end of the femur, a spot of calcification that develops late in pregnancy, appeared later in growth-restricted babies, at a median of about 33.6 weeks versus 32.1 weeks in controls. This suggests that growth restriction affects not just bone length but the pace of bone maturation itself, which is information a simple length measurement on ultrasound would miss entirely.