A 20-week pregnant woman undergoes routine obstetric ultrasound. The sonographer measures the biparietal diameter (BPD), head circumference (HC), femur length (FL), and abdominal circumference (AC). All of the following statements regarding second-trimester biometry are correct EXCEPT:

Explanation

## Correct Answer Analysis **High-Yield:** The femur length (FL) measurement technique is critical for accurate dating. The FL must be measured from the **proximal femoral metaphysis (just below the greater trochanter) to the distal femoral metaphysis (just above the lateral femoral condyle)**—NOT from the greater trochanter to the lateral condyle as stated in option 1. ## Second-Trimester Biometric Standards | Parameter | Measurement Level | Accuracy | Notes | |-----------|-------------------|----------|-------| | **BPD** | Widest transverse diameter at level of thalami | ±3–5 mm | Outer edge to outer edge | | **HC** | Calculated from BPD and OFD | ±3–5 mm | More accurate than BPD alone | | **FL** | Proximal metaphysis to distal metaphysis | ±3–5 mm | NOT from trochanter to condyle | | **AC** | At level of stomach + umbilical vein | ±5–8 mm | Outer edge to outer edge | ## Key Point: **Warning:** A common pitfall is measuring FL from bony landmark to bony landmark (trochanter to condyle) rather than from metaphysis to metaphysis. This introduces error because: - The greater trochanter is a lateral projection and not the true proximal femoral endpoint - The lateral femoral condyle is not the distal endpoint—the measurement should end at the distal metaphysis - This leads to **overestimation of FL** and incorrect dating ## Clinical Pearl: **Key Point:** In the second trimester (14–20 weeks), all four parameters (BPD, HC, FL, AC) have similar accuracy (±3–5 mm). After 20 weeks, accuracy decreases progressively, making early dating critical. ## Correct Statements Verified - **Option 0 (BPD accuracy):** ✓ Correct — BPD ±3–5 mm in 2nd trimester - **Option 2 (HC calculation):** ✓ Correct — HC = π × (BPD + OFD)/4, more accurate than BPD alone - **Option 3 (AC level):** ✓ Correct — AC measured at junction of umbilical vein and stomach - **Option 1 (FL measurement):** ✗ **INCORRECT** — FL measured from proximal to distal metaphysis, NOT from trochanter to condyle