Single Best Answer (SBA) · Obstetric Ultrasound
A clinician schedules the anomaly scan. Which single timing and purpose is correct?
Show answer and explanation
Answer: A. At 18-22 weeks, to survey fetal structure and localise the placenta
The anomaly scan (USS2)
| Routine ultrasound | Timing and purpose |
|---|---|
| Dating scan (USS1) | 10-13+6 weeks; crown-rump length (CRL) for dating; viability, number/chorionicity; nuchal translucency |
| Anomaly scan (USS2) | 18-22 weeks; structural survey; placental localisation |
| Growth scans (USS3) | Serial (e.g. 28, 32, 36 weeks); AC, HC, BPD, FL -> estimated fetal weight |
| Delivery planning (USS4) | Presentation, placenta, liquor near term |
Timing and purpose
- Performed at 18-22 weeks.
- It surveys the structure of major organs (including the heart) and localises the placenta.
Placental localisation
- About 15-20% of women have a low-lying placenta at 20 weeks, but most 'migrate' upward as the lower segment forms; only about 10% of these turn out to have placenta praevia.
Cardiac detection
- The routine scan has limited sensitivity for major cardiac defects; specialist echocardiography detects more in high-risk pregnancies.
So the anomaly scan is at 18-22 weeks for the structural survey and placental localisation.
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