Synopsis
Select and interpret antenatal ultrasound by gestation and indication while explaining screening limits, escalating placental or fetal concerns and avoiding unsupported routine scanning.
- Offer the routine dating scan at 11+2 to 14+1 weeks to establish gestational age, identify multiple pregnancy and, if chosen, support combined trisomy screening.
- Use crown-rump length for first-trimester dating within the programme range; record chorionicity and amnionicity at the first reliable scan in a multiple pregnancy.
- Offer the fetal anomaly screening scan at 18+0 to 20+6 weeks; it examines anatomy for 11 programme conditions but cannot identify every structural, genetic or developmental disorder.
- When image quality prevents completion, arrange the programme-permitted repeat promptly so the screening pathway is completed by 23+0 weeks where possible.
- From 24+0 weeks, measure symphysis-fundal height at routine contacts in a singleton pregnancy unless serial ultrasound growth surveillance is already planned; do not measure more frequently than every 2 weeks.
- Offer ultrasound for a small fundal-height measurement and determine urgency using movements, blood pressure, growth history and other risk; consider ultrasound when the measurement is large.
- Do not offer routine ultrasound after 28 weeks to an uncomplicated singleton pregnancy solely for reassurance; later scans are risk- or finding-directed.
- Interpret estimated fetal weight as an estimate with error, using centile, abdominal circumference, growth trajectory, amniotic fluid and Doppler rather than one isolated number.
Key red flags
An inability to demonstrate fetal cardiac activity when it should be visible requires prompt expert confirmation and a sensitive pregnancy-loss pathway.
Reduced fetal movement must be assessed immediately through the gestation-appropriate pathway; a booked growth scan is not a substitute for cardiotocography or triage.
Estimated fetal weight or abdominal circumference below the 10th centile, falling growth velocity or abnormal umbilical-artery Doppler needs an FGR surveillance and timing-of-birth plan.
Major placenta praevia, suspected accreta spectrum, vasa praevia or significant bleeding requires specialist placental assessment and delivery planning.
A major structural anomaly, markedly increased nuchal translucency or discordant growth in a multiple pregnancy warrants timely fetal-medicine referral.
Severe hypertension, headache, visual symptoms, epigastric pain or proteinuria with growth concern requires same-day maternal and fetal evaluation.
Investigation priorities
Assign gestational age and estimated date of delivery during the validated first-trimester size range.
Management branches
The pregnancy reaches the national 11+2 to 14+1 week scan window or earlier dating remains uncertain.
- Confirm viability and measure crown-rump length using the programme method, reconciling menstrual or assisted-conception dates appropriately.
- Determine fetal number and document chorionicity and amnionicity, obtaining senior review promptly if membrane or placental features are uncertain.
- If trisomy screening is accepted and technically eligible, complete nuchal translucency and the linked blood-test pathway without conflating it with dating alone.