How we estimate it
Count back 266 days
If you were born on your due date, you were conceived about 266 days before your birthday. We show that day.
Born early or late
If you know you were born early or late, enter the week you were born for a better estimate.
The possible range
Most babies are born between 37 and 42 weeks of pregnancy, so we also show the full possible range.
This is a fun estimate, not a fact about your family. The moon phase is accurate to about a day.
Why sites give different dates
Some sites count 280 days back from a birthday, others 266. 280 days is counted from the mother’s last period; conception is about 2 weeks later, so from conception it is about 266 days. We use 266.
Questions people ask
How accurate is it?
It gives a likely window of about 5 weeks. Babies are born at different times, so nobody can know the exact day from a birthday alone.
Why do some sites count 280 days?
280 days is counted from the mother’s last period. Conception is about 2 weeks later, so from conception to birth it is about 266 days.
I was born early or late. Does that change it?
Yes. Enter the week you were born. Each week earlier moves the likely date 7 days later, and each week later moves it 7 days earlier.
Was I conceived around Christmas?
If you were born on time around 17 to 23 September, counting back 266 days lands close to Christmas or New Year. It is a fun estimate, not a fact.
Can I use it for my child?
Yes. It works for any birthday. For a pregnancy happening now, use the conception date calculator.
Read the full guide: When was I conceived?
Sources
- ACOG Committee Opinion 579: Definition of Term Pregnancy · checked October 24, 2026
- ACOG Committee Opinion 700: Methods for Estimating the Due Date · checked October 24, 2026