The scans
| Scan | Timing | Purpose |
|---|---|---|
| Early pregnancy (TVS) | 5–10 weeks | location (intrauterine vs ectopic), viability, number and chorionicity (λ/twin‑peak = dichorionic; T‑sign = monochorionic), dating by CRL (most accurate dating parameter: ± 5–7 days) |
| NT scan | 11 + 0 to 13 + 6 weeks (CRL 45–84 mm) | nuchal translucency (≥ 3.5 mm or > 99th centile abnormal: aneuploidy, cardiac defects, genetic syndromes), nasal bone, ductus venosus flow, tricuspid regurgitation; combined test (NT + age + free β‑hCG + PAPP‑A); early structural anomalies (anencephaly, abdominal wall defects, megacystis) |
| Anomaly (level II/TIFFA) scan | 18–22 weeks | systematic survey: head (lemon and banana signs: spina bifida), face (cleft), spine, heart (4‑chamber, outflow tracts), abdomen (double bubble, gastroschisis vs omphalocele), kidneys, limbs, placenta, cord, liquor; soft markers |
| Growth / wellbeing | 3rd trimester | biometry (BPD, HC, AC, FL: AC most sensitive for growth restriction), estimated fetal weight, AFI/deepest vertical pocket, Doppler (umbilical artery: raised PI → absent/reversed EDF = severe placental insufficiency; MCA PI fall: brain‑sparing; MCA PSV for fetal anaemia, e.g. Rh isoimmunisation), biophysical profile, presentation, placental location |