A 28-year-old woman presents to the antenatal clinic with a 6-week history of amenorrhea and mild nausea. Urine beta-hCG is positive. She is unsure of her last menstrual period. What is the investigation of choice to confirm pregnancy and establish accurate gestational age?

Explanation

## Diagnosis and Dating of Early Pregnancy ### Role of Ultrasound in Early Pregnancy **Key Point:** Transvaginal ultrasound (TVS) is the gold standard for confirming pregnancy and establishing accurate gestational age in the first trimester, particularly when the menstrual history is uncertain. ### Advantages of Transvaginal Ultrasound | Feature | Transvaginal | Transabdominal | Serum hCG | |---------|--------------|-----------------|----------| | **Gestational sac visibility** | 4.5–5 weeks | 5–6 weeks | Cannot date | | **Yolk sac visibility** | 5–5.5 weeks | 6 weeks | Cannot date | | **Fetal pole visibility** | 5.5–6 weeks | 6–6.5 weeks | Cannot date | | **Dating accuracy** | ±3–5 days | ±3–5 days | None | | **Confirms intrauterine pregnancy** | Yes, earliest | Yes, later | No | **High-Yield:** In the first trimester, transvaginal ultrasound can detect pregnancy 1–2 weeks earlier than transabdominal ultrasound and provides the most accurate dating (±3–5 days). This is crucial when the LMP is uncertain or irregular. ### Why TVS Is Superior Here 1. **Confirms intrauterine pregnancy** — rules out ectopic pregnancy (critical safety issue). 2. **Earliest detection** — gestational sac visible at 4.5–5 weeks of gestation. 3. **Accurate dating** — crown-rump length (CRL) measurement in first trimester has ±3–5 days accuracy. 4. **Detects viability** — fetal heart rate visible by 5.5–6 weeks. **Clinical Pearl:** A positive urine hCG does not confirm intrauterine pregnancy or provide dating information. Serum hCG quantification can suggest gestational age by hCG levels, but it is less accurate and does not visualize the pregnancy. Transabdominal ultrasound is acceptable but inferior to TVS in early pregnancy due to later visualization and slightly lower accuracy. **Mnemonic:** **TVUS** = **T**ransvaginal **U**ltrasound **S**uperior (earliest, most accurate, confirms intrauterine location). [cite:Williams Obstetrics 26e Ch 8]