The Femur Length Head Circumference (FL/HC) Ratio Calculator helps healthcare professionals assess fetal growth patterns by comparing the length of the fetus’s femur bone to the circumference of its head. This ratio plays a critical role in prenatal screenings and can signal potential developmental issues. A lower-than-normal ratio may indicate conditions such as skeletal dysplasia or intrauterine growth restriction (IUGR). This tool supports early diagnosis and better pregnancy management.
formula
FL/HC Ratio = Femur Length / Head Circumference
Where:
- Femur Length (FL): The length of the fetal femur measured via ultrasound (in mm or cm)
- Head Circumference (HC): The circumference of the fetal head (in the same unit as FL)
The result is unitless, since both values use the same measurement unit.
Supporting Notes:
- The FL/HC ratio should be interpreted with gestational age in mind.
- Normal values increase gradually throughout pregnancy.
- A low ratio (e.g., < 0.17 in later trimesters) may raise concerns about disproportionate growth or skeletal anomalies.
- A high ratio is less common but may still prompt further clinical investigation.
Table: Reference FL/HC Ratios by Gestational Age
| Gestational Age (Weeks) | Typical Femur Length (mm) | Typical Head Circumference (mm) | FL/HC Ratio |
|---|---|---|---|
| 18 | 28 | 150 | 0.187 |
| 22 | 38 | 200 | 0.190 |
| 26 | 48 | 250 | 0.192 |
| 30 | 58 | 285 | 0.203 |
| 34 | 66 | 315 | 0.210 |
| 38 | 72 | 330 | 0.218 |
Note: These values are average estimates and should be interpreted with clinical context.
Example
Scenario:
- Femur Length = 52 mm
- Head Circumference = 280 mm
FL/HC Ratio = 52 / 280 = 0.186
Interpretation:
At around 28–30 weeks, a ratio of 0.186 is slightly below average and may prompt closer monitoring for fetal growth patterns.
Most Common FAQs
A: It depends on gestational age. Around the third trimester, a ratio near 0.20–0.22 is typical.
A: It helps detect signs of disproportionate fetal growth, which can point to conditions like IUGR or skeletal abnormalities.
A: No. It only provides supportive data. A doctor must interpret the results in combination with other clinical findings and ultrasound measurements.