Obstetric Topic 45. Ultrasound diagnosis during gestation

I. General Principles

Role of Ultrasound

• Main imaging method in pregnancy → confirms pregnancy, dates gestation, assesses fetal anatomy/growth, placenta, amniotic fluid and fetal well-being.
• May guide invasive procedures: amniocentesis, CVS, cordocentesis.
• Hungarian-style exam sequence: 0, 1st, 2nd, 3rd and optional 4th screening US.
• Technique: transvaginal US is best early; transabdominal US is standard later; Doppler is used when flowmetry is needed.

II. 0 Screening Ultrasound

Timing and Aims

• Timing: GW6-7, usually TVS.
• Confirm pregnancy location → intrauterine vs ectopic pregnancy.
• Detect fetal heart activity and fetal pole when gestational age is sufficient.
• Detect multiple pregnancy.
• Intrauterine yolk sac is more reliable than gestational sac alone for excluding ectopic pregnancy.

III. 1st Screening Ultrasound

Timing and Core Measurements

• Timing: GW11-13.
• Detect fetal heart activity and number of fetuses; determine chorionicity/amnionicity if multiple pregnancy.
• Crown-rump length (CRL): most accurate 1st-trimester dating method.
• Nuchal translucency (NT): normally <3 mm.
• Increased NT → ↑ risk of chromosomal abnormalities, e.g. Down syndrome, and cardiac defects.
• Nasal bone: absent/hypoplastic nasal bone increases trisomy risk.
• Abnormal NT or structural anomaly → genetic counseling + diagnostic testing offer.

IV. 2nd Screening Ultrasound

Timing and Aims

• Timing: GW18-22; classic anatomy/genetic scan.
• Biometry: BPD, OFD, HC, AC, FL → estimated fetal weight and growth.
• Detect congenital malformations and soft markers of chromosomal abnormalities.
• Assess fetal organs, placenta location/previa, amniotic fluid (AFI or SDP) and fetal position.
• Fetal echocardiography if cardiac view is abnormal or risk is high.

V. 3rd Screening Ultrasound

Timing and Aims

• Timing: GW28-32.
• Biometry → estimated fetal weight and diagnosis/surveillance of IUGR/FGR.
• Flowmetry: umbilical artery, middle cerebral artery (MCA), uterine artery.
• Assess fetal organs, amniotic fluid, placental location/maturation and fetal presentation.
• MCA Doppler may show brain-sparing in fetal hypoxia/FGR.

VI. 4th / Late Ultrasound

Timing and Aims

• Timing: ~GW36-38 when used; not universal in every protocol.
• Delivery planning: fetal presentation, estimated fetal weight, placental location/maturation and amniotic fluid.
• Biophysical profile (BPP) may be used to assess fetal well-being.

Biophysical Profile (BPP)

• Components: NST + fetal breathing + gross movement + fetal tone + amniotic fluid.
• Scoring: 0 or 2 points each → total 10.
• ≥8 with normal amniotic fluid → reassuring fetal well-being.
• ≤6, or 8 with oligohydramnios → abnormal/equivocal.
• ≤4 → severe fetal compromise/distress possible → urgent management.

VII. Practical Exam Points

• Dating: 1st-trimester CRL is best; later dating is less accurate.
• 18-22 weeks: key scan for structural anomalies.
• Growth assessment requires serial measurements, not a single number alone.
• US detects many but not all malformations; normal US does not exclude genetic or functional disease.