How it is calculated
- From last period: gestational age = target date − first day of last period.
- From ultrasound: age on target date = age at the scan + days since the scan.
- Embryonic (fetal) age = gestational age − about two weeks.
A first-trimester ultrasound usually dates pregnancy to within ±5–7 days.
Worked example
A scan on 1 June gave 12 weeks 2 days: on 1 September (92 days later) you are 25 weeks 3 days.
Frequently asked questions
Gestational vs embryonic age?
Gestational age counts from the last period; embryonic age from conception, about two weeks less.
Which do I use if the scan and LMP differ?
If the difference is significant, doctors usually use first-trimester ultrasound dating.
What is preterm birth?
Birth before 37 completed weeks of gestation.