Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Dating, anomaly and growth ultrasound
Obstetrics and gynaecology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
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Abnormal fetal wellbeing overrides the routine scan timetable
Absent fetal cardiac activity, severe growth restriction with abnormal Doppler, reduced fetal movement, major bleeding, maternal hypertension or suspected membrane rupture requires urgent clinical assessment rather than waiting for the next scheduled ultrasound.
Action: Assess maternal observations and symptoms, confirm fetal viability and wellbeing through maternity triage, and involve obstetric or fetal-medicine services immediately when ultrasound or Doppler suggests compromise.
Purpose and principles
Select and interpret antenatal ultrasound by gestation and indication while explaining screening limits, escalating placental or fetal concerns and avoiding unsupported routine scanning.
Ultrasound has different purposes at different stages, and a report should be interpreted against the question that prompted it. A viability scan, first-trimester dating scan, 20-week anomaly screen and third-trimester growth assessment are not interchangeable. Before requesting or interpreting a scan, confirm gestation, pregnancy symptoms, previous findings and the decision the result will change. Explain that ultrasound is non-ionising and has no known harm when clinically used, but it remains a screening and measurement tool with biological and technical limitations.
NICE offers a routine scan between 11+2 and 14+1 weeks. Its core purposes are dating the pregnancy and identifying plurality; if the patient accepts trisomy screening, nuchal translucency is added within the combined-test pathway. Crown-rump length is the preferred dating measurement while the fetus is within the validated size range. Dating based on a high-quality early ultrasound is generally more reliable than a last menstrual period affected by irregular cycles, uncertain recall, recent contraception or bleeding, and the assigned estimated date of delivery should not be repeatedly changed after small later discrepancies.
Indications, selection and cautions
Reliable first-trimester dating
A technically adequate crown-rump-length measurement in the validated window establishes gestation more securely than an uncertain menstrual date.
Incomplete anomaly screen
Required anatomy has not been visualised because of position or image limitation, which is different from an abnormal structure being seen.
Suspected fetal growth restriction
Small abdominal circumference or estimated weight, slowed trajectory or placental Doppler abnormality raises concern that pathological growth limitation is present.
Large measurement pattern
Fundal height or biometry above the expected range can reflect inaccurate dates, constitution, diabetes-related growth, excess fluid, fibroids or plurality.
Placental imaging concern
Low placental location, lacunae, abnormal interface or exposed fetal vessels may require targeted transvaginal and specialist imaging for birth planning.
Red flags requiring action
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.
Method and interpretation
01Crown-rump-length dating ultrasoundFirst step
Why
Assign gestational age and estimated date of delivery during the validated first-trimester size range.
Interpretation and limitations
Use the programme measurement standard and avoid redating from minor later differences, which may represent growth variation rather than incorrect dates.
02First-trimester plurality assessment
Why
Count fetuses and determine chorionicity and amnionicity while placental and membrane signs are clearest.
Interpretation and limitations
Shared chorion or amnion increases specific fetal risks and changes scan frequency, referral and delivery planning.
03FASP 20-week screening scan
Why
Screen systematically for 11 specified physical conditions between 18+0 and 20+6 weeks.
Interpretation and limitations
A suspected condition leads to expert review and possible diagnostic testing; a normal screen cannot exclude every fetal disorder.
04Symphysis-fundal height
Why
Provide serial clinical surveillance of singleton growth after 24 weeks when routine ultrasound surveillance is not already in place.
Interpretation and limitations
Plot the measurement and use trajectory; small size prompts ultrasound, while maternal habitus, fibroids, fluid and lie can reduce accuracy.
05
Clinical next steps
01First trimesterDate the pregnancy and define pluralityFirst stepThe pregnancy reaches the national 11+2 to 14+1 week scan window or earlier dating remains uncertain.+
1Confirm viability and measure crown-rump length using the programme method, reconciling menstrual or assisted-conception dates appropriately.
2Determine fetal number and document chorionicity and amnionicity, obtaining senior review promptly if membrane or placental features are uncertain.
3If trisomy screening is accepted and technically eligible, complete nuchal translucency and the linked blood-test pathway without conflating it with dating alone.
02Anatomy screenComplete and communicate the 20-week examinationThe pregnancy is within 18+0 to 20+6 weeks and the patient chooses fetal anomaly screening.+
1Perform the standard anatomical views and record placental location, explaining beforehand that the programme covers selected conditions rather than every anomaly.
Risks, monitoring and follow-up
Retain the assigned estimated date of delivery and the measurement used so later teams do not repeatedly redate normal biological variation.
For multiple pregnancy, keep chorionicity visible in every handover because it controls the scan schedule and complication pathway.
Document whether the anomaly scan was complete, repeated, declined or remained incomplete and confirm that any referral result returned.
Plot fundal-height and ultrasound measurements consistently, noting factors that reduce accuracy and avoiding falsely precise interpretation of estimated weight.
When serial growth surveillance is indicated, use the pathway interval and include movements, maternal observations, fluid and Doppler rather than ordering isolated reassurance scans.
Safety-net every apparently reassuring scan with immediate maternity contact for reduced movement, bleeding, fluid loss, severe pain or pre-eclampsia symptoms.
Special situations
Dating and growth answer different questions
Early size assigns gestation; later deviation from that assigned gestation may represent growth change and should not automatically rewrite the due date.
Incomplete is not abnormal
Poor visualisation commonly reflects position or technical limits, but a timely repeat matters because an unseen structure has not been screened.
Trajectory can reveal late FGR
A fetus may remain above the 10th centile while abdominal growth slows, so absolute centile alone can miss placental deterioration.
Chorionicity beats zygosity clinically
Whether twins share a placenta determines key antenatal risks and surveillance even when genetic similarity is not yet known.
A normal scan has boundaries
Ultrasound can reduce the chance of selected abnormalities but cannot exclude microscopic, biochemical, later-developing or poorly visualised disease.
Common pitfalls
01
Calling the 20-week scan a complete fetal health check or promising that a normal result excludes genetic disease.
02
Redating a pregnancy repeatedly from third-trimester biometry and thereby hiding genuine growth restriction.
03
Ordering routine late ultrasound for every uncomplicated singleton pregnancy despite the NICE recommendation against it.
04
Waiting for the next scan when reduced fetal movement or severe hypertension needs immediate assessment.
05
Labelling a fetus growth restricted from one estimated-weight value without considering interval growth, Doppler and constitutional context.
06
Failing to establish chorionicity in the first trimester and assuming it can be reconstructed reliably near delivery.
Practice
Two practice questions
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Question 1 of 2
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Obstetrics and gynaecologyOriginal SBA
Purpose of the dating scan
A patient attends at 12+4 weeks with uncertain last menstrual period dates and has accepted combined trisomy screening. What should the routine ultrasound appointment establish?
Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Multiple pregnancy transforms the first-trimester scan. Record number of fetuses, placental masses, membrane characteristics, chorionicity and amnionicity because shared placentation determines surveillance and complications. If chorionicity is uncertain, seek senior imaging review promptly rather than assigning it later when membrane signs are less clear. Assisted-conception dating follows embryo-transfer or ovulation information within the fertility pathway; do not overwrite a reliable ART date casually with later biometry.
The NHS Fetal Anomaly Screening Programme offers the 20-week scan at 18+0 to 20+6 weeks. A systematic examination looks for 11 specified physical conditions and assesses structures including brain, face, heart, spine, abdomen, kidneys and limbs. It can also identify placental location and markers that require follow-up. A normal result does not promise a healthy baby: some conditions cannot be seen, develop later, are outside the programme or are obscured by fetal position, maternal tissues or equipment limits.
When the sonographer cannot complete required views, explain the reason without implying abnormality and arrange the one programme repeat within the permitted pathway, aiming to complete screening by 23+0 weeks. Persistently incomplete imaging after the repeat is documented and managed according to which anatomy remains unseen and the overall risk; repeated non-programme scans are not automatically useful. An unexpected suspected condition requires prompt referral, clear written information and diagnostic choices, with space for uncertainty rather than a definitive label from a screening image.
Growth surveillance begins with risk assessment at booking and again in the second trimester. In an uncomplicated singleton pregnancy not already having serial scans, measure symphysis-fundal height after 24+0 weeks at routine visits, separated by at least 2 weeks. Plot the measurement consistently and consider trend, not only one point. A small measurement prompts ultrasound, with urgency increased by reduced movement, hypertension or other placental-risk features. A large measurement can reflect dates, constitution, fibroids, fluid, multiple pregnancy or accelerated growth and should lead to a targeted assessment where appropriate.
A growth scan measures head, abdominal and femur parameters to derive an estimated fetal weight and evaluates anatomy visible at that gestation. The indication determines whether to add amniotic-fluid assessment, umbilical-artery Doppler, middle-cerebral-artery Doppler, placental assessment or cervical information. Measurement error grows in late pregnancy, so a single centile is not a birthweight guarantee. Abdominal circumference below the 10th centile, estimated weight below the 10th centile or a declining trajectory can indicate placental insufficiency even when the absolute weight remains above the 10th centile.
NICE advises against routine ultrasound after 28 weeks in an uncomplicated singleton pregnancy. This avoids false reassurance, incidental findings and interventions without demonstrated benefit. Later imaging is appropriate for suspected size abnormality, reduced movements with an indication, hypertension, diabetes, previous growth restriction or stillbirth, placental concerns, presentation uncertainty or another risk-defined pathway. When FGR is diagnosed, surveillance and timing of birth use fetal movement, maternal blood pressure and proteinuria, growth, fluid, cardiotocography and Doppler together; an isolated ultrasound estimate never replaces the whole assessment.
Key points
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.
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.
Ultrasound fetal biometry
Why
Estimate fetal size and growth trajectory when clinical measurement or baseline risk justifies imaging.
Interpretation and limitations
Interpret abdominal circumference and estimated-weight centiles with interval growth and measurement error; one result does not distinguish constitutionally small from FGR.
06Amniotic fluid and fetal Doppler
Why
Assess placental function and fetal adaptation when growth restriction or compromise is suspected.
Interpretation and limitations
Reduced fluid or abnormal umbilical-artery and other indicated Dopplers increases concern and may alter surveillance intensity and timing of birth.
2Arrange the permitted repeat rapidly when required views are incomplete, aiming to finish the pathway by 23+0 weeks and documenting residual limitations.
3For a suspected abnormality, provide prompt fetal-medicine review, balanced information and appropriate diagnostic options rather than presenting screening as certainty.
03Growth concernMove from clinical signal to placental assessmentFundal height, history, maternal disease, fetal movement or prior imaging raises concern about fetal size or growth.+
1Assess movements, maternal blood pressure and urine protein immediately and choose urgency according to the combined maternal and fetal risk.
2Obtain ultrasound biometry with fluid, placental and Doppler assessment matched to the indication, comparing with reliable prior measurements at a meaningful interval.
3If FGR or another abnormality is suspected, enter the specialist surveillance and birth-timing pathway; if normal, state the appropriate ongoing clinical or scan follow-up.