## 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]
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