First Trimester Ultrasound

Aka: First Trimester Ultrasound, Ultrasound Pregnancy Dating, Gestational Sac, Yolk Sac, Embryo, Crown-Rump Length

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II. 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)

III. Findings: First Trimester Ultrasound

  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

IV. Exam: First Trimester (Transvaginal Ultrasound)

  1. Gestational Sac (5 weeks gestation, bHCG 1800)
    1. Round echolucent (black) sac with echogenic ring located in fundus
    2. Age on mean diameter: (length + width + height)/3
  2. Yolk Sac (6 weeks gestation)
    1. Small echogenic walled round sac within Gestational Sac
    2. May appear more as '=' sign rather than circle
    3. Presence rules out pseudogestational sac of ectopic
  3. Embryo (7 weeks gestation, bHCG 20,000)
    1. Onset as fetal pole dot on the Yolk Sac edge
    2. Grows at 1 mm per day
    3. Fetal heart activity by Crown-Rump Length 5 mm
  4. Crown-Rump Length most reliable dating measure
    1. Menstrual age (weeks) = Crown-Rump Length (cm) + 6.5
    2. Table of menstrual ages per Crown-Rump Length
      1. 6.1 Weeks: 0.4 cm
      2. 7.2 Weeks: 1.0 cm
      3. 8.0 Weeks: 1.6 cm
      4. 9.2 Weeks: 2.5 cm
      5. 9.9 Weeks: 3.0 cm
      6. 10.9 Weeks: 4.0 cm
      7. 12.1 Weeks: 5.5 cm
      8. 13.2 Weeks: 7.0 cm
      9. 14.0 Weeks: 8.0 cm

V. References

  1. Paspulati (2004) Radiol Clin North Am 42(2):297-314 [PubMed]

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