Lecture Notes: First Trimester Pregnancy Ultrasound

Introduction

• Presenter: Ajit Panag, Emergency Ultrasound Fellow
• Topic: First Trimester Pregnancy Ultrasound
• Outline:
  • Indications
  • Anatomy Review
  • Probe Type
  • Technique
  • Pathology

Indications for First Trimester Ultrasound

• Not performed on all first trimester pregnancies.
• Indicated with:
  • Positive pregnancy test and:
    • Pelvic or abdominal pain
    • Vaginal bleeding
    • Dizziness or syncope
    • Pelvic mass
• Clinical Questions:
  • Confirm intrauterine pregnancy vs. ectopic pregnancy
  • Assess for pregnancy viability or loss
  • Identify other findings (e.g., gestational trophoblastic disease)

Key Definitions

• Gestational Age: Measured from the first day of the last menstrual period; conception age plus two weeks.
• Discriminatory Zone:
  • HCG level indicating visualization of gestational sac.
  • Transvaginal: 1,200-1,500
  • Transabdominal: 5,000-6,000

Anatomy Review

• Uterus Position: Between bladder and recto sigmoid colon; anti-flexed at a 90-degree angle with vaginal canal.
• Fallopian Tubes & Ovaries: Enter uterus at lateral cornea.
• Cul-de-sacs:
  • Anterior: Between bladder and uterus; potential space.
  • Posterior (Pouch of Douglas): Between uterus and colon; checks for pelvic fluid pooling.

Probe Type

• Transabdominal Ultrasound:
  • Uses low-frequency curvilinear probe.
  • Provides broader view of uterus and adnexa.
• Transvaginal Ultrasound (Endocavitary):
  • More detailed images; less common in emergency settings.

Technique and Positioning

• Patient Position: Supine for transabdominal; full bladder needed.
• Transducer Position:
  • Midline sagittal view first, just above pubic bone.
  • Followed by midline transverse view.

Protocol Steps

1. Midline Sagittal View:
   • Transducer on midline abdominal wall, just above pubic bone.
   • Identify uterus with midline stripe, cervix, and vaginal stripe.
2. Midline Transverse View:
   • Turn probe 90 degrees counterclockwise.
   • Shows transverse uterus sections from cervix to fundus.

Findings and Pathology

• Intrauterine vs. Ectopic Pregnancy:
  • Gestational Sac: First visible structure.
    • Includes double decidual sign (not diagnostic).
  • Pseudogestational Sac: Mistaken identity in ectopic cases.
• Yolk Sac: First definitive evidence of intrauterine pregnancy.
• Fetal Pole/Embryo: Visible by about 6-7 weeks.
• Fetal Cardiac Activity:
  • Use M-mode over Doppler for safety.
  • Normal fetal heart rates provided by gestational age.

Ectopic Pregnancy

• Hard to diagnose via transabdominal ultrasound.
• Signs:
  • Tubal Ring Sign: Hyperechoic ring around tubal mass.
  • Ring of Fire Sign: High vascularity around ectopic.

Additional Considerations

• Molar Pregnancy: Snowstorm appearance; high HCG levels.
• Twins: Visible with two fetal structures.

Key Takeaways

• Always rule in intrauterine pregnancy with any pregnant patient.
• Ultrasound is crucial, especially for hemodynamically unstable patients.