The femur length (FL) measurement refers to the thigh bone, the longest bone in the body. This measurement is a standard part of prenatal ultrasound examinations, providing insight into the longitudinal growth of the developing fetus.
Femur Length Chart by Week
Fetal femur length measurements provide an average reference for growth at different gestational weeks. These measurements are obtained during ultrasound scans and are presented here in centimeters. Individual fetal growth can vary, and these values represent typical averages based on population studies.
| Gestational Week | Average Femur Length (cm) |
| :————— | :———————— |
| 14 | 2.0 |
| 18 | 3.0 |
| 22 | 4.0 |
| 26 | 5.0 |
| 30 | 6.0 |
| 34 | 6.7 |
| 38 | 7.5 |
| 40 | 7.8 |
These figures are derived from established reference ranges, such as those used by The Fetal Medicine Foundation. They serve as a guide for healthcare providers to track the fetus’s growth trajectory.
Purpose of Measuring Fetal Femur Length
Measuring fetal femur length is a part of fetal biometry, a comprehensive assessment of fetal size through ultrasound. This measurement is taken alongside other parameters, including biparietal diameter (BPD), head circumference (HC), and abdominal circumference (AC). These combined measurements contribute to an overall picture of fetal development.
The primary purpose of these measurements is to estimate the gestational age of the fetus, especially when the last menstrual period is uncertain. They also help assess the fetus’s overall growth rate and estimated weight. By tracking these dimensions over time, healthcare providers can monitor whether the fetus is growing as expected for its gestational age.
Understanding Variations in Femur Length
It is common for a fetus’s femur length measurement to be somewhat above or below the average for a given gestational week. Such variations are often benign and influenced by several factors. Parental height and genetic background play a role, as taller parents may naturally have fetuses with longer femurs.
Ethnicity can also contribute to slight differences in average fetal measurements. Technical aspects of the ultrasound scan, such as the fetus’s position or movement during the examination, can affect measurement accuracy. These minor deviations do not indicate a concern.
A significant deviation from the average might prompt a healthcare provider to conduct a closer review. This further assessment helps screen for certain conditions that could affect fetal growth. Femur length measurement is a screening tool, not a diagnostic one. Any concerns about specific results should be discussed with a doctor or midwife, who can provide context based on the complete clinical picture.