A 28-year-old woman presents with amenorrhea of 6 weeks. Urine beta-hCG is positive. She has no risk factors for ectopic pregnancy. What is the investigation of choice to confirm intrauterine pregnancy and establish accurate dating?

Question

Accepted Answer

A. Transvaginal ultrasound. ## Investigation of Choice for Confirming Intrauterine Pregnancy and Dating

**Key Point:** Transvaginal ultrasound (TVS) is the gold standard for early pregnancy confirmation and accurate dating in the first trimester.

### Why Transvaginal Ultrasound?

1. **Earliest detection:** Gestational sac visible at hCG levels of 1,000–2,000 mIU/mL (around 4.5–5 weeks)
2. **Superior resolution:** High-frequency probe (7–10 MHz) provides excellent visualization of early intrauterine pregnancy
3. **Accurate dating:** Crown-rump length (CRL) measurement has ±3–5 days accuracy in first trimester
4. **Rules out ectopic pregnancy:** Confirms intrauterine location, critical for safe management
5. **Non-invasive, safe:** No radiation, no contrast agents

### Comparison of Dating Methods

| Investigation | Timing | Accuracy | Purpose |
|---|---|---|---|
| **Transvaginal ultrasound** | 4.5–14 weeks | ±3–5 days | Gold standard for dating & location |
| Serum beta-hCG quantitation | Qualitative only | Cannot date | Confirms pregnancy, not dating |
| Transabdominal ultrasound | 5–14 weeks | ±1–2 weeks | Less sensitive in early pregnancy |
| Pelvic examination | Clinical | Unreliable | Confirms pregnancy clinically, not accurate dating |

**High-Yield:** In the first trimester (≤13 weeks + 6 days), TVS with CRL measurement is the most accurate method for pregnancy dating, with error margin of only ±3–5 days. This dating should NOT be changed by later scans.

**Clinical Pearl:** Even in low-risk patients, TVS is essential to exclude ectopic pregnancy, which can present with positive hCG and amenorrhea but no intrauterine gestational sac.

[cite:Williams Obstetrics 26e Ch 5]

Answer

B. Pelvic examination

Answer

C. Transabdominal ultrasound

Answer

D. Serum beta-hCG quantitation

A 28-year-old woman presents with amenorrhea of 6 weeks. Urine beta-hCG is positive. She has no risk factors for ectopic pregnancy. What is the investigation of choice to confirm intrauterine pregnancy and establish accurate dating?

Explanation

Investigation of Choice for Confirming Intrauterine Pregnancy and Dating

Why Transvaginal Ultrasound?

Comparison of Dating Methods

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Investigation	Timing	Accuracy	Purpose
Transvaginal ultrasound	4.5–14 weeks	±3–5 days	Gold standard for dating & location
Serum beta-hCG quantitation	Qualitative only	Cannot date	Confirms pregnancy, not dating
Transabdominal ultrasound	5–14 weeks	±1–2 weeks	Less sensitive in early pregnancy
Pelvic examination	Clinical	Unreliable	Confirms pregnancy clinically, not accurate dating