Your due date is 280 days (40 weeks) after the first day of your last period, or 266 days after you conceived. Enter either date to see the due date, how many weeks along you are, and your trimester.
The formula
From the last period
due date = first day of last period + 280 days + (cycle length − 28)From conception
due date = conception date + 266 daysHow far along
gestational age = as-of date − (due date − 280 days)Both methods count 40 weeks from the same point, the day a 28-day cycle would have begun. Weeks and days are then the gestational age divided by 7. The method follows ACOG Committee Opinion 700, Methods for Estimating the Due Date.
Worked example
With a 32-day cycle instead, the starting point moves four days later, and the due date becomes 21 October 2026.
When to use it, and the mistakes to avoid
Use it as soon as you know the date of your last period or of conception: to plan appointments, to tell the people who need to know, and to see which stage you are at.
- Treating it as a deadline. It is the middle of a range, not a date the baby is due to arrive.
- Ignoring your cycle length. A 35-day cycle puts the due date a week later than the standard 28-day assumption.
- Counting from the wrong day. The first day of bleeding is day one, not the day it ended.
- Overriding your doctor. An early scan is a better guide than any calculator. Use their date.
- Using it for fertility treatment. Dates for IVF and other treatment are set from the treatment itself, so go by your clinic's.
FAQ
How is the due date calculated from the last period?
By Naegele's rule: add 280 days (40 weeks) to the first day of your last period. With the usual 28-day cycle, 10 January becomes 17 October. If your cycle is longer or shorter than 28 days, the date moves later or earlier by the same number of days, which is why the calculator asks for your cycle length.
How is it calculated from the conception date?
Add 266 days (38 weeks) to the conception date. That is the same date as the last-period method gives for a 28-day cycle, because conception normally happens about two weeks after the period begins.
How are the trimesters divided?
The first trimester runs from the start to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth. Your week is counted from the dating start, not from conception, so you are already counted as two weeks pregnant on the day you conceive.
How likely is it that my baby arrives on the due date?
Not very. Only a minority of babies are born on their due date; most arrive in the weeks either side of it. Full term is 39 weeks 0 days to 40 weeks 6 days, so being a week early or late is entirely ordinary.
Why might my doctor give a different due date?
Because an ultrasound scan in the first trimester measures the baby and is a more accurate guide to dating than a remembered period, especially if your cycles are irregular. If a scan puts the date more than a few days from the calculator's, your doctor will usually use the scan date. Follow theirs.