The measurement of the fetal femur length (FL) is a standard procedure in prenatal care, offering valuable insight into a developing baby’s growth trajectory. FL measures the thigh bone, the longest bone in the body, and is one of several biometric parameters obtained during a routine obstetric ultrasound examination. This measurement estimates overall fetal size and, particularly in the second trimester, helps confirm the gestational age of the pregnancy. Tracking this bone length allows healthcare providers to monitor growth patterns and ensure the baby is developing as expected.
How Femur Length is Determined
Obtaining an accurate femur length measurement is a precise technical process performed by a sonographer or physician during the ultrasound scan. The goal is to visualize the entire length of the bone’s shaft, known as the diaphysis, in a clean, longitudinal view. The ultrasound beam is aligned perpendicular to the bone to capture the clearest possible image, which typically appears as a bright, white structure on the screen.
The measurement involves placing electronic calipers at the blunt ends of the ossified bone, from the proximal end near the hip to the distal end near the knee. It is important to measure only the hardened bone shaft, meticulously excluding the cartilaginous growth plates, or epiphyses, at either end. This measurement is recorded in millimeters or centimeters and is included with other standard biometry, such as the biparietal diameter (BPD) and abdominal circumference (AC), to complete the fetal growth assessment.
Standard Femur Length Ranges by Week
For most pregnancies, the fetal femur length follows a predictable pattern of growth, allowing healthcare providers to compare a baby’s measurement against reference charts. These charts establish the expected range of lengths for each gestational week, typically represented by percentiles, with the 50th percentile being the average. Tracking these numbers ensures the fetus remains within the normal range of growth for its age.
The progression from the mid-second trimester to near-term demonstrates a steady, linear increase in length. Around 18 weeks of gestation, the average femur length is approximately 2.6 centimeters. By 24 weeks, the expected length increases to roughly 3.8 centimeters.
Growth continues rapidly through the third trimester, reflecting the baby’s overall size increase. At the 30-week mark, the typical measurement reaches about 5.2 centimeters. Closer to the expected delivery date, around 36 weeks, the average femur length is near 6.0 centimeters. Measurements falling between the 10th and 90th percentile are usually considered well within the normal range.
Clinical Significance of the Measurement
The femur length measurement serves a broad purpose in determining overall fetal health and development. Its primary utility is confirming the gestational age of the pregnancy, especially when initial dating is uncertain. In the second trimester, the FL measurement is one of the most reliable predictors of gestational age.
Along with the head and abdominal circumference, femur length is a key component in formulas used to calculate the Estimated Fetal Weight (EFW). The EFW represents the baby’s overall size, which is a significant factor in managing the remainder of the pregnancy and planning for delivery. Monitoring the EFW percentile over time helps to identify babies who may be growing too slowly or too quickly.
Regular assessment of femur length throughout the second and third trimesters monitors the baby’s growth trajectory. A consistent measurement within the normal range indicates proportional skeletal development and adequate nutrition. If femur growth slows relative to the head and abdomen, it alerts the care team to potential issues that warrant further investigation. This proportional assessment evaluates the effectiveness of the placental environment.
Understanding Deviations and Follow-Up
When femur length falls significantly outside the expected range, typically below the 5th percentile, it is referred to as a short femur length (SFL) and requires clinical review. While an isolated SFL often relates to the baby’s genetic background, it is also recognized as a soft marker for certain conditions. SFL can be a sign of skeletal dysplasia, which affects bone and cartilage growth, or it may be associated with chromosomal conditions such as Trisomy 21.
A short femur length can also indicate Intrauterine Growth Restriction (IUGR) or a baby that is Small for Gestational Age (SGA). Follow-up typically involves a detailed, extended organ screening to look for other subtle abnormalities. If other anomalies are detected, genetic counseling and further diagnostic testing may be offered to the parents.
If the short femur is found in isolation, without any other markers, the approach shifts to intensified monitoring of the pregnancy. This includes serial ultrasounds every two to four weeks to track the growth rate and Doppler assessments to check blood flow in the umbilical artery and other vessels. This close observation helps distinguish between a healthy, constitutionally small baby and one experiencing growth restriction due to placental function.