A 28-year-old primigravida presents to the antenatal clinic with amenorrhoea of 6 weeks. Urine beta-hCG is positive. On examination, the uterus is just palpable above the symphysis pubis. She reports irregular menstrual cycles (28–35 days) in the past. What is the most appropriate next step in management?

Explanation

## Clinical Context A positive urine hCG with 6 weeks of amenorrhoea and an irregular menstrual history requires **accurate dating** before proceeding with routine antenatal care. Irregular cycles make last menstrual period (LMP)–based dating unreliable. ## Why Transvaginal Ultrasound Is the Best Next Step **Key Point:** Transvaginal ultrasound is the gold standard for **first-trimester dating and viability assessment** in the 6–12 week window. **High-Yield:** At 6 weeks gestation (by LMP): - Gestational sac is visible on transvaginal US (typically from 4.5–5 weeks) - Yolk sac appears at ~5.5 weeks - Fetal pole with cardiac activity appears at ~6 weeks **Clinical Pearl:** In women with irregular cycles, transvaginal ultrasound dating in the first trimester has an accuracy of ±3–5 days, far superior to LMP dating (±2 weeks). This establishes the **expected date of delivery (EDD)** and guides all subsequent screening and monitoring. ## Management Sequence After Confirmation | Step | Timing | Purpose | |------|--------|----------| | Transvaginal US | Now (6 weeks) | Confirm IUP, rule out ectopic, establish dating | | Routine antenatal care | 8–12 weeks | Booking visit, baseline investigations | | First-trimester screening | 11–14 weeks | Combined screening (NT, PAPP-A, beta-hCG) | | Folic acid | Start now | 5 mg daily for 12 weeks (higher dose for irregular cycles / PCOS) | **Tip:** Do NOT delay ultrasound confirmation in women with irregular cycles—LMP-based dating will lead to incorrect EDD and inappropriate timing of subsequent investigations.