A 32-year-old woman attends the antenatal clinic at 14 weeks of gestation (by LMP). Transvaginal ultrasound performed at 8 weeks had confirmed a single intrauterine pregnancy with normal cardiac activity. Today, repeat transabdominal ultrasound shows a crown-rump length (CRL) of 72 mm, which corresponds to 12 weeks of gestation. She is asymptomatic with no vaginal bleeding. What is the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis A discrepancy between LMP-based dating (14 weeks) and ultrasound dating (12 weeks by CRL at 14 weeks clinically) has emerged. The earlier ultrasound at 8 weeks confirmed viability; the current ultrasound shows a 2-week lag in CRL. ## Dating Accuracy and Revision Rules **Key Point:** Ultrasound dating accuracy varies by trimester: - **First trimester (≤13 weeks):** ±3–5 days - **Second trimester (14–20 weeks):** ±1–2 weeks - **Third trimester (>20 weeks):** ±3–4 weeks **High-Yield:** When there is a **discrepancy between LMP and ultrasound in the second trimester:** - If the difference is **≤2 weeks:** Accept the ultrasound dating (more accurate than LMP, especially if LMP is uncertain or cycles are irregular) - If the difference is **>2 weeks:** Investigate for growth restriction or dating error **Clinical Pearl:** In this case, the 2-week discrepancy is at the threshold. However, because: 1. The earlier 8-week scan confirmed viability and normal growth trajectory 2. The woman is asymptomatic with no bleeding 3. The CRL at 14 weeks clinically (72 mm = 12 weeks) is only a 2-week lag **The most appropriate action is to revise the EDD based on the current ultrasound** (now dating to 12 weeks, pushing EDD forward by 2 weeks) and continue routine care. This is NOT intrauterine growth restriction (which would show progressive lag across serial scans or abnormal Doppler). ## When to Escalate | Finding | Action | |---------|--------| | Discrepancy ≤2 weeks, no other concerns | Revise EDD, continue routine care | | Discrepancy >2 weeks OR progressive lag on serial scans | Investigate for IUGR, assess Doppler | | Abnormal anatomy on detailed survey | Refer to fetal medicine | | Oligohydramnios or other red flags | Escalate to maternal–fetal medicine | **Tip:** Dating revisions in the second trimester are common and do not automatically signal pathology. The key is serial assessment—if growth remains appropriate on follow-up scans, the revised dating is correct.