17/10/2025
OBSTETRIC ULTRASOUND 🫄
On a prenatal (obstetric) ultrasound, FL and AC are two key biometric parameters used to assess fetal growth and estimate gestational age.
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🦴 FL – Femur Length
Definition: Measurement of the length of the fetal femur (thigh bone).
Purpose:
Estimates fetal age and growth.
Helps assess skeletal development.
Abnormally short FL may suggest growth restriction or skeletal dysplasia.
Measurement Technique:
Taken from one end of the femoral shaft to the other (excluding the femoral head and distal epiphysis).
Normal Appearance:
A bright (echogenic) linear structure, usually measured in the longitudinal plane.
Correlation:
FL correlates well with gestational age after 14 weeks.
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⚪ AC – Abdominal Circumference
Definition: Measurement around the outer perimeter of the fetal abdomen.
Purpose:
One of the most important parameters for assessing fetal weight and growth pattern.
Sensitive indicator of intrauterine growth restriction (IUGR) or macrosomia.
Measurement Technique:
Taken on a transverse section of the fetal abdomen at the level of:
Stomach bubble
Portal sinus of the umbilical vein
The spine should appear as a short segment, and ribs should form a symmetric circle.
Used for:
Estimated Fetal Weight (EFW)
Growth percentile calculations (with BPD, HC, FL)
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📏 Typical Use in Combination
Parameter Meaning Helps Estimate
BPD Biparietal Diameter Head size
HC Head Circumference Brain growth
AC Abdominal Circumference Body and liver growth
FL Femur Length Skeletal growth
Together, these are used to calculate:
Gestational age
Estimated fetal weight (EFW)
Growth trends over time