Ultrasound in Pregnancy — OBG Overview

1. Introduction & Safety

2. Types of Obstetric Ultrasound Examinations

3. First-Trimester Ultrasound (< 14 Weeks)

Key Objectives

• Confirm intrauterine pregnancy and exclude ectopic
• Establish gestational age — crown-rump length (CRL) is the most accurate biometric parameter in the first trimester
• Determine fetal cardiac activity
• Assess fetal number and chorionicity in multiples (most accurately determined before 14 weeks)
• Nuchal translucency (NT) measurement at 11–13+6 weeks for Down syndrome and other aneuploidies screening
• Detect major structural anomalies (with emerging evidence for late first-trimester detailed scans)

Crown-Rump Length (CRL)

• Most reliable dating method; accurate to ±5–7 days
• Obtained in a true neutral position — fetus neither hyperflexed nor hyperextended

Nuchal Translucency

• The fluid-filled space at the back of the fetal neck
• NT ≥ 3.0 mm is associated with increased risk of trisomy 21, 18, 13, cardiac defects, and other structural anomalies
• Combined with serum free β-hCG and PAPP-A (combined first-trimester screening) for detection rates of ~85–90%

4. Midtrimester Anatomy Scan (18–22 Weeks) — The "Anatomy Scan"

• Confirming/correcting gestational age by comparing obstetric dates with fetal biometry
• Detecting structural anomalies before fetal viability
• Identifying placental abnormalities (previa, accreta spectrum)
• Detecting uterine abnormalities (fibroids, etc.)

Fetal Biometry (Standard Measurements)

Figure: Head circumference (HC) and BPD measurement

Detailed Fetal Anatomy Checklist (CPT 76805)

• Cranial bones, falx cerebri, cavum septi pellucidi
• Thalami, lateral ventricles (normal < 10 mm)
• Cerebellum, cisterna magna
• Orbits, facial profile, nasal bone, upper lip

Figure: Cavum septum pellucidum (arrow)

Figure: Posterior fossa — cerebellum and nuchal fold measurement

• Four-chamber view of the heart
• Cardiac axis (normally ~45° to the left)
• Left and right ventricular outflow tracts
• Lung parenchyma

Figure: Four-chamber heart view in diastole and systole (RV, LV, RA, LA)

Figure: Left ventricular outflow tract

• Stomach (should be visible and fluid-filled)
• Abdominal umbilical cord insertion
• Kidneys and bladder
• Umbilical arteries (should be two)

• Spine in longitudinal and transverse views
• All four extremities (upper and lower long bones)
• Hand/foot anatomy and posture
• Genitalia

Figure: Fetal profile with nasal bone (marker for Down syndrome screening)

Placenta, Cord & Amniotic Fluid

• Location (note if low-lying or previa)
• Appearance (masses, cysts, lucencies, accessory lobe)
• Umbilical cord insertion site (central/marginal/velamentous)
• Number of umbilical cord vessels (normally 3 — 2 arteries + 1 vein)

• Maximum Vertical Pocket (MVP): Normal 2–8 cm
• Amniotic Fluid Index (AFI): Sum of MVPs in four quadrants; normal 5–24 cm
• Oligohydramnios: MVP < 2 cm / AFI < 5 cm
• Polyhydramnios: MVP > 8 cm / AFI > 24 cm

5. Third-Trimester Ultrasound — Growth & Well-being

Fetal Growth Assessment

• Serial biometry every 3–4 weeks in growth-restricted fetuses
• EFW < 10th percentile = small for gestational age (SGA); EFW < 3rd percentile = severely growth-restricted

Biophysical Profile (BPP)

6. Doppler Velocimetry

Umbilical Artery (UA) Doppler

• Reflects placental vascular resistance
• Normal: diastolic flow present and progressive throughout pregnancy
• Abnormal patterns (in order of severity):
  1. Elevated S/D ratio / increased PI, RI — increased resistance
  2. Absent End-Diastolic Velocity (AEDV) — critical worsening
  3. Reversed End-Diastolic Velocity (REDV) — imminent fetal compromise; strong indicator for delivery

Figure: Progressive umbilical artery resistance — from nearly normal at 18 weeks to absent end-diastolic velocities at 24 weeks

Middle Cerebral Artery (MCA) Doppler

• Measures MCA Peak Systolic Velocity (PSV)
• Brain-sparing effect: in FGR, MCA resistance decreases (vasodilation) as the fetus preferentially perfuses the brain
• MCA PSV > 1.5 MoM (multiples of the median): hallmark for fetal anaemia (e.g., Rh isoimmunization, hydrops fetalis)

Ductus Venosus (DV) Doppler

• Reflects right heart pressure and venous return
• Absent or reversed a-wave in DV waveform = sign of severe cardiac compromise; indicates imminent fetal acidosis and guides timing of delivery in preterm FGR

7. Cervical Length Assessment

• Transvaginal ultrasound (TVS) is the standard method
• Short cervix (< 25 mm before 24 weeks) predicts preterm birth
• Used to guide decisions on:
  • Cervical cerclage
  • Progesterone supplementation
  • Antenatal corticosteroids

8. Ultrasound Markers for Aneuploidy

First Trimester Markers

Second Trimester "Soft Markers"

9. Special Applications

Multiple Pregnancy

• Chorionicity determination is critical and most accurately assessed at < 14 weeks:
  • Dichorionic-diamniotic (DCDA): "Twin peak" (lambda sign) — triangular wedge of placental tissue between membranes
  • Monochorionic-diamniotic (MCDA): "T-sign" — membrane inserts at right angle to placenta
• Ultrasound staging of Twin-Twin Transfusion Syndrome (TTTS) using Quintero criteria (based on UA, UV, DV Doppler and amniotic fluid discordance)
• Detection of Selective FGR in monochorionic twins

Placental Abnormalities

• Placenta previa: Placenta covering the internal os — typically confirmed by TVS at 32–36 weeks
• Placenta accreta spectrum: Abnormal placentation suspected on grey-scale and colour Doppler — loss of retroplacental clear space, placental lacunae, bridging vessels to bladder

10. Reporting Requirements

• Patient identifying information and date
• Indication for the procedure and type of examination
• Gestational age by established dates
• Number of fetuses (and chorionicity if multiples)
• Fetal biometry with predicted gestational age
• Principal findings (normal, abnormal, or poorly visualized)
• Differential diagnosis for abnormalities
• Recommendations for management or follow-up

Summary Table: Ultrasound Schedule in Normal Pregnancy