Which of the following is the most accurate method for pregnancy dating in the second trimester?

Question

Accepted Answer

B. Head circumference (HC) with ±2 weeks accuracy. ## Accuracy of Biometric Parameters for Pregnancy Dating

**Key Point:** Head circumference (HC) is the most accurate single biometric parameter for dating in the second trimester, with an accuracy margin of ±2 weeks.

### Comparison of Biometric Accuracy by Trimester

| Parameter | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|
| Crown-rump length (CRL) | ±3–5 days | Not used | Not used |
| Biparietal diameter (BPD) | — | ±2–3 weeks | ±3–4 weeks |
| Head circumference (HC) | — | ±2 weeks | ±3–4 weeks |
| Femur length (FL) | — | ±3–4 weeks | ±4–5 weeks |
| Abdominal circumference (AC) | — | ±4–5 weeks | ±5–6 weeks |

**High-Yield:** The accuracy of all biometric parameters **decreases with advancing gestational age**. CRL in the first trimester is the gold standard (±3–5 days), but in the second trimester, HC becomes the most accurate single parameter.

### Clinical Application

**Clinical Pearl:** When multiple parameters are available:
  - Use the **average of 2–3 parameters** for better accuracy
  - HC and BPD are preferred over FL and AC in the second trimester
  - AC is the least reliable for dating and is better used for growth assessment

**Mnemonic:** **CHFA** — **C**RL (1st trim), **H**C (2nd trim), **F**L (3rd trim), **A**C (growth, not dating)

**Tip:** In clinical practice, if dating by ultrasound differs from LMP by >2 weeks in the second trimester, the ultrasound date is preferred.

Answer

A. Femur length (FL) with ±4 weeks accuracy

Answer

C. Biparietal diameter (BPD) with ±3 weeks accuracy

Answer

D. Abdominal circumference (AC) with ±5 weeks accuracy

Which of the following is the most accurate method for pregnancy dating in the second trimester?

Explanation

Accuracy of Biometric Parameters for Pregnancy Dating

Comparison of Biometric Accuracy by Trimester

Clinical Application

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Parameter	1st Trimester	2nd Trimester	3rd Trimester
Crown-rump length (CRL)	±3–5 days	Not used	Not used
Biparietal diameter (BPD)	—	±2–3 weeks	±3–4 weeks
Head circumference (HC)	—	±2 weeks	±3–4 weeks
Femur length (FL)	—	±3–4 weeks	±4–5 weeks
Abdominal circumference (AC)	—	±4–5 weeks	±5–6 weeks