Significance of Femur Length in Pregnancy

Closeup shot of a doctor showing a patient ultrasound scans on a digital tablet in her office
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Early in pregnancy, certain ultrasound findings such as the presence of a yolk sac and the crown-to-rump ratio are used to help determine the health of a pregnancy, the gestational age, and the possibility for pregnancy loss. After the first trimester, the embryo has developed into a fetus and new markers are used to narrow in on gestational age and assess the health of the baby.

Femur Length Measurement

Among the markers used for assessing fetal growth and health is the length of the baby's femur, the long bone in the thigh. Measured from the blunt end of the bone to the shaft, the femur length is generally measured in millimeters.

A short femur length finding on ultrasound may indicate the need for further testing to rule out certain conditions. But it is also important to keep in mind the limitations inherent to using femur length as a marker for poor pregnancy outcomes.

When femur length is below the fifth percentile, parents may be advised about a number of potential atypical pregnancy outcomes. A short femur length identified on ultrasound in the second or third trimester raises concern for the conditions detailed below.

But, this measurement has many limitations, from human error to outdated ultrasound equipment to normal variation. Femur length is only one variable among many that should be used to make determinations about a baby's health.

It is important to know that in the majority of these pregnancies (73%), parents will go on to have a full-term delivery of a healthy baby whose size is appropriate for gestational age.

Dwarfism

Fetuses with shorter-than-expected femur length have been found to be at higher risk for skeletal dysplasia, otherwise known as dwarfism. This is different from short stature, which is a height that is three or more standard deviations below the mean for age but is proportional.

There are more than 200 disorders that can be categorized as skeletal dysplasia. All are characterized by a disproportionate skeleton due to cartilage and bone growth abnormalities.

Placental Insufficiency

Other studies have raised concerns about the sufficiency of the placenta to provide an adequate nutritional environment for fetuses with short femur lengths. Because of that, short femur length is linked to other adverse pregnancy outcomes, such as fetuses that are small for gestational age, babies born with low birth weight, and preterm birth.

Aneuploidies

A femur length that is shorter than expected can also be a soft marker for certain genetic conditions, such as trisomy 21 (Down syndrome), trisomy 13 (Patau syndrome), and trisomy 18 (Edward's syndrome). Soft markers are indicators found on ultrasounds that are not abnormalities on their own, but characteristics that occur more often in fetuses with chromosomal trisomies.

Compared to high-level markers such as nuchal skin fold, femur length is considered a low-level marker for Down syndrome. The presence of any of these markers may simply indicate a need for more prenatal testing.

A Word From Verywell

It can be frightening to hear that your baby has a short femur length, or another marker of concern. However, in most cases, the outcome is still a healthy, full-term baby. If your doctor finds that your baby has a shorter than expected femur length, further testing may help you and your healthcare provider understand what this measurement means and how best to care for your baby during pregnancy, at birth, and beyond.

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