10 - pregnancy dating

1. Definitions
1. Actual Fetal age dated from time of conception
2. Menstrual Age (gestational age) = Conception + 14 days
2. Naegele's Rule for calculating EDC
1. Start with the First Day of LMP
2. Add 7 days
3. Subtract 3 months
3. 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
4. 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
5. Radiology: Obstetric Ultrasound
1. Accurate to +/- 8% of estimated gestational age