## Accuracy of Second-Trimester Biometric Parameters **Key Point:** In the second trimester (13–24 weeks), BPD and FL are the primary dating parameters, with accuracy of ±3–5 mm. HC is derived from BPD and becomes increasingly useful. AC is NOT a reliable dating tool in the second trimester because it is highly variable and influenced by maternal nutrition and fetal growth patterns. ### Biometric Accuracy Hierarchy by Trimester | Parameter | First Trimester | Second Trimester | Third Trimester | |-----------|-----------------|------------------|------------------| | **CRL** | ±3–5 days | — | — | | **BPD** | — | ±3–5 mm (±1 week) | ±15–20 mm (±3–4 weeks) | | **FL** | — | ±3–5 mm (±1 week) | ±15–20 mm (±3–4 weeks) | | **HC** | — | ±5 mm (±1–2 weeks) | ±20 mm (±3–4 weeks) | | **AC** | — | ±10 mm (±2–3 weeks) | ±20 mm (±4–5 weeks) | **High-Yield:** AC becomes the most accurate parameter only in the **third trimester** (after 24 weeks) for growth assessment, not dating. In the second trimester, AC is too variable to be used for accurate gestational age estimation. **Clinical Pearl:** The rule of thumb is: **earlier in pregnancy = smaller structures = greater accuracy**. As pregnancy advances, biometric variability increases, so dating accuracy decreases. **Warning:** Confusing AC as a dating tool in the second trimester is a common trap. AC is used for growth assessment and detecting intrauterine growth restriction (IUGR), not for accurate dating in mid-pregnancy. ### Why Each Parameter Works or Doesn't Work for Dating - **BPD & FL (Second Trimester):** Minimal genetic and nutritional variation; highly reproducible; ±1 week accuracy. - **HC:** Derived from BPD; similarly accurate in second trimester. - **AC (Second Trimester):** Highly influenced by liver size, amniotic fluid volume, and maternal factors; poor dating accuracy. [cite:Callen Ultrasound in Obstetrics and Gynecology 6e Ch 3]
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