Obstetrics Book


First Trimester Ultrasound

Aka: First Trimester Ultrasound, Ultrasound Pregnancy Dating, Gestational Sac, Yolk Sac, Embryo, Crown-Rump Length
  1. See Also
    1. Obstetric Ultrasound
    2. Second Trimester Ultrasound
    3. Antenatal Screening
    4. Trisomy Findings on Fetal Ultrasound
    5. Nuchal Translucency
  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. 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
  4. 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
  5. References
    1. Paspulati (2004) Radiol Clin North Am 42(2):297-314 [PubMed]

Gestational Sac (C0553498)

Definition (MSH) Round-shaped structure usually located in the upper fundus of the UTERUS in early pregnancy. It comprises EMBRYO; AMNIOTIC FLUID; and YOLK SAC. It is visible in PRENATAL ULTRASONOGRAPHY by the fifth week of pregnancy. Its size is often used to determine and monitor GESTATIONAL AGE; FETAL GROWTH; and PREGNANCY COMPLICATIONS.
Definition (NCI) The sac that envelops the embryo. It is composed of extraembryonic membranes.
Concepts Embryonic Structure (T018)
MSH D058746
LNC LP7273-8, MTHU002950
English Gestational sac, Sacs, Gestational, Gestation Sacs, Gestational Sacs, Gestation Sac, Sacs, Gestation, Gestational Sac, Sac, Gestation, Sac, Gestational, sac gestational, gestational sac
French Sac de gestation, Sac gestationnel
German Gestationssack
Czech gestační váček
Italian Sacco gestazionale
Spanish Saco Gestacional
Portuguese Saco Gestacional
Polish Pęcherzyk ciążowy
Norwegian Gestasjonssekk, Fosterblære, Saccus gestationalis, Blastocyste
Derived from the NIH UMLS (Unified Medical Language System)

Entire embryo (C1305370)

Concepts Embryonic Structure (T018)
SnomedCT 57991002
English Embryo, Entire embryo (body structure), Entire embryo, Embryo, NOS
Spanish embrión [como un todo] (estructura corporal), embrión [como un todo], embrión
Derived from the NIH UMLS (Unified Medical Language System)

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