Reverse Due Date Calculator
Know the due date but not the dates behind it? Work backwards to your last period, conception day, and today's exact week.
Standard obstetric arithmetic in reverse: last period = due date − 280 days, conception = due date − 266 days (ACOG Committee Opinion 700).
Questions people ask
Why does this say my last period was before I conceived?
Gestational age is counted from the first day of the last period, which is about two weeks before conception — that is why 'week 4' begins around when a test first turns positive.
My provider gave me a different date after ultrasound.
Ultrasound dating — especially in the first trimester — is more accurate than calendar math, and providers often revise the due date after it. Always follow the revised date.