Obstetric Ultrasound

1. Indications
   1. Determination of gestational age
   2. Diagnosis
      1. Suspected Miscarriage or fetal demise
      2. Vaginal Bleeding
      3. Pelvic Pain
      4. Suspected Multiple Gestation
      5. Suspected hydatiform mole
      6. Suspected Ectopic Pregnancy
      7. Size-date discrepancy
      8. Uterine or pelvic mass or abnormality
      9. Congenital anomalies
      10. Fetal Presentation
   3. Antenatal Monitoring
      1. Biophysical Profile (BPP)
      2. Intrauterine Growth Retardation (IUGR)
      3. Fetal Macrosomia
   4. Adjunct to obstetrical procedures
      1. Chorionic Villus Sampling
      2. Amniocentesis
      3. Cephalic Version
   5. ACOG recommends Ultrasound at 18 weeks for all patients
      1. Confirm dates and fetal survey
2. Interpretation: Gestational Age related findings
   1. Transvaginal Ultrasound
      1. Days 35-37: Gestational Sac of 5 mm (bHCG 1500-2000)
      2. Days 37-40: Yolk Sac (Gestation sac>9-10 mm, confirms intrauterine pregnancy)
      3. Day 40: Fetal Pole (Gestational Sac>16-18 mm, bHCG 5000)
      4. Day 45: Fetal Heart Activity (Crown rump length >5 mm, bHCG 17,000)
   2. Abdominal Ultrasound
      1. Day 42: Gestational Sac (bHCG 3500)
      2. Days 49-50: Embryo 5-10 mm with cardiac activity (bHCG 20,000)
3. Efficacy
   1. Radius Study evaluated screening ultrasounds
      1. Evaluated low risk patients (n=15,151)
      2. Found no benefit
         1. No difference in perinatal mortality
         2. No difference in birthweight or preterm birth
      3. Did not evaluate high risk patients or other benefits
   2. Risks of screening ultrasounds
      1. False positives
         1. Anxiety
         2. Unnecessary intervention
      2. False negatives
         1. False sense of reassurance
      3. Expensive: $200-$300
      4. Patients now want recreational U/S or for sexing
      5. Safety
         1. Thermal index (heat) should be 2 Celsius or less
            1. Increased risk with doppler ultrasound
         2. Mechanical index should be 4 or less
         3. Possible association with Speech Delay in children
            1. Does not appear to be significant risk
   3. Benefits of screening:
      1. Accurate gestational age
      2. Earlier detection (first trimester) of fetal anomaly
      3. Enhanced prenatal bonding
4. Physics:
   1. Resolution increases with frequency
   2. Penetration decreases with increased frequency
      1. Transabdominal: 3.5 Mhz to 5.0 Mhz
      2. Transvaginal: 5.0 Mhz to 7.5 Mhz
   3. Piezoelectric crystal
      1. Apply a mechanical force sends out electric signal
      2. Apply an electric current, sends out sound
5. First Trimester Ultrasound Findings
   1. Gestational Sac Location
      1. Intrauterine Pregnancy
      2. Ectopic Pregnancy
   2. Identification of Embryo
      1. Assess for Multiple Gestation
   3. Evaluation of Adnexa
      1. Corpus Luteal Cysts
   4. Menstrual age
      1. Gestational Sac Size
      2. Crown Rump Length
   5. Viability
      1. Fetal Heart beat
      2. Growth of Gestational Sac
   6. Early detection of fetal anomaly
      1. Anencephaly
      2. Cystic Hygroma
6. Second and Third Trimester Ultrasound
   1. All Ultrasounds:
      1. Fetal Presentation
      2. Fetal Viability
      3. Fetal Biometry
      4. Fetal Survey
      5. Evaluation of amniotic fluid volume
   2. Level I Ultrasound: standard exam
      1. Biometry
      2. Fetal survey
   3. Level II Ultrasound: High risk
      1. Usually performed by perinatologist
7. Fetal Survey
   1. Intracranial Anatomy
      1. Start with biparietal diameter view
      2. Rotate transducer 20-30 degrees
         1. Still transects thalamus
         2. Now transects Cerebellum
         3. Also transects cisterna magna and nuchal thickness
      3. Evaluate structures
         1. Cerebellum
         2. Choroid plexus
         3. Cisterna magna
         4. Lateral Ventricles
         5. Cavum septum pellucidum
      4. Assess for meningomyelocele
      5. Image
         1. 
   2. Chest including four chamber heart
      1. Critical to confirm that septum is not off axis
      2. Assess left and right outflow tracts
      3. Images
         1. 
   3. Stomach location
      1. Assess for Diaphragmatic Hernia
   4. Diaphragm
      1. 
   5. Kidneys and Bladder
      1. Use lower longitudinal spine view to start
      2. Scan anterior to spine to capture Kidneys
   6. Cord insertion and three vessel cord
      1. 
   7. Spine
      1. Cervical spine
      2. Thoracic Spine
      3. Lumbosacral spine
   8. Extremities
      1. Evaluate arms, legs and hands
8. References
   1. Paspulati (2004) Radiol Clin North Am 42(2):297-314