Pregnancy Dating (Gestational Age Calculation)
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There are 2 main ways to calculate gestational age.
1. Dating by Last Menstrual Period
Gestational age is calculated from the first day of the last menstrual period.
This assumes:
- Regular 28-day cycle, and
- Ovulation on day 14
Calculation:
- Gestational age = current date – last menstrual period
Naegle’s rule is used to calculate the estimated date of delivery (EDD):
- EDD = last menstrual period + 1 year – 3 months + 7 days
2. Dating Scan (Ultrasound) – Gold Standard
The dating scan is offered at 11-14 weeks
- The crown-rump length is measured on ultrasound
- The machine then automatically converts it into gestational age (based on a standardised reference dataset / chart)
If the gestational age estimated from the last menstrual period and the dating scan differ → use the gestational age from the dating scan