Femur Length Head Circumference Ratio Calculator

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.

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