How to estimate gestation age

• From the Last menstrual period (LMP)and using Nagle's rule

- Provided the menses is regular
- Need to adjust if usual period is more or less than 28 days

• Using Ultrasound scan

- Measuring the crown rump length (week 7 to week 14)
- Parietal diameter or femur length (Week 14 to week 20)
- Not useful for dating beyond 20 weeks
- More accurate than LMP