Free Pregnancy Tool
Gestational Age (AOG)
Calculate age of gestation (AOG) on any date, from the last period, the due date, or an earlier ultrasound.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: AOG = (chosen date − first day of last period) in weeks and days. From an ultrasound: AOG at scan + days since the scan.
- From the last period: count the days from the first day of the last period to the chosen date.
- From a due date: the start is the due date minus 280 days.
- From an ultrasound: take the gestational age reported at the scan and add the days since.
- Choose any date, past or future, to see the AOG then.
Everything is worked out in your browser. Belly Weekly does not send or store your answers.
Questions
What does AOG mean?
Age of gestation: how far along a pregnancy is, written in weeks and days.
Is gestational age the same as the baby's age?
No. Gestational age counts from the last period, about 2 weeks before conception.
Which date should I use?
If you had an early ultrasound, its dating is usually the one your care team uses.