II. Definitions

  1. Actual Fetal age dated from time of conception
  2. Menstrual Age (Gestational age) = Conception + 14 days

III. History: Naegele's Rule for calculating ED+C

  1. Start with the First Day of LMP
  2. Add 7 days
  3. Subtract 3 months

IV. History: Pregnancy history accuracy for dating

  1. In vitro fertilization: accurate to +/- 1 day
  2. Single recorded Intercourse accurate to +/- 3 days
  3. Basal Body Temp Record accurate to +/- 4 days
  4. Recorded first day of LMP accurate to +/- 2-3 weeks
  5. Quickening accurate to +/- 4-6 weeks

V. Exam

  1. Fundal Height
    1. Indicated for pregnancy beyond 20 weeks gestation
    2. Accurate to 4-6 weeks gestation
  2. Detection Fetal Heart Tones accurate to +/- 4-6 weeks
    1. Doppler (first heard at 7-12 weeks)
    2. Fetoscope (first heard at 18-21 weeks)
  3. First trimester bimanual exam accurate to +/- 1-2 weeks
    1. Assumes normal Uterus

VI. Imaging: Obstetric Ultrasound

  1. Accurate to +/- 8% of estimated Gestational age
  2. Indications to change due date based on Ultrasound results
    1. Ultrasound at <9 weeks gestation
      1. Ultrasound due date differs from LMP-based dates by 5 days or more
    2. Ultrasound at 9 to 16 weeks gestation
      1. Ultrasound due date differs from LMP-based dates by 7 days or more
    3. Ultrasound at 16 to 22 weeks gestation
      1. Ultrasound due date differs from LMP-based dates by 10 days or more
    4. Ultrasound at 22 to 28 weeks gestation
      1. Ultrasound due date differs from LMP-based dates by 14 days or more
    5. Ultrasound at >28 weeks gestation
      1. Ultrasound due date differs from LMP-based dates by 21 days or more
  3. References
    1. (2014) Obstetric Gynecol 124(4):863-6 +PMID:25244460 [PubMed]

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