II. Definitions
- Actual Fetal age dated from time of conception
- Menstrual Age (Gestational age) = Conception + 14 days
III. History: Naegele's Rule for calculating ED+C
- Start with the First Day of LMP
- Add 7 days
- Subtract 3 months
IV. History: Pregnancy history accuracy for dating
- In vitro fertilization: accurate to +/- 1 day
- Single recorded Intercourse accurate to +/- 3 days
- Basal Body Temp Record accurate to +/- 4 days
- Recorded first day of LMP accurate to +/- 2-3 weeks
- Quickening accurate to +/- 4-6 weeks
V. Exam
- Fundal Height
- Indicated for pregnancy beyond 20 weeks gestation
- Accurate to 4-6 weeks gestation
- Detection Fetal Heart Tones accurate to +/- 4-6 weeks
- Doppler (first heard at 7-12 weeks)
- Fetoscope (first heard at 18-21 weeks)
- First trimester bimanual exam accurate to +/- 1-2 weeks
- Assumes normal Uterus
VI. Imaging: Obstetric Ultrasound
- Accurate to +/- 8% of estimated Gestational age
- Indications to change due date based on Ultrasound results
- Ultrasound at <9 weeks gestation
- Ultrasound due date differs from LMP-based dates by 5 days or more
- Ultrasound at 9 to 16 weeks gestation
- Ultrasound due date differs from LMP-based dates by 7 days or more
- Ultrasound at 16 to 22 weeks gestation
- Ultrasound due date differs from LMP-based dates by 10 days or more
- Ultrasound at 22 to 28 weeks gestation
- Ultrasound due date differs from LMP-based dates by 14 days or more
- Ultrasound at >28 weeks gestation
- Ultrasound due date differs from LMP-based dates by 21 days or more
- Ultrasound at <9 weeks gestation
- References