How this calculator works
Clinical dating commonly treats pregnancy as 40 weeks, or 280 days, from the first day of the last menstrual period. This page takes the due date you enter, subtracts 280 days to estimate that starting date, and then counts from it to the date you choose. Gestational age is that elapsed count in weeks and days; the trimester boundaries are 14 weeks and 28 weeks of gestational age. The week markers list the calendar date on which each standard week begins. This is a countdown, not a prediction: dating from an early ultrasound takes precedence when it differs, and births do not arrive on a fixed schedule. It is not medical advice.
Formula
estimated LMP = due date - 280 days. gestational days = count date - estimated LMP. days remaining = due date - count date. Gestational age = floor(gestational days / 7) weeks and the remainder in days. First trimester runs through 13 weeks 6 days, second through 27 weeks 6 days, and the third begins at 28 weeks.
Worked example
For a due date of 2027-02-01 counted from 2026-11-01, 92 days remain, which is 13 weeks and 1 day to go. The gestational age is 26 weeks and 6 days, in the second trimester, and the estimated last menstrual period is 2026-04-27. The 28-week marker falls on 2026-11-09.
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Frequently asked questions
Is the due date a prediction of the birth day?
No. It is a dating estimate based on a 40-week convention, and few births occur exactly on that date. An early ultrasound can move the estimate, and your care team's dating should be used instead when it differs.
Why does the countdown start before conception?
Gestational age is dated from the last menstrual period rather than the uncertain day of fertilisation. About two of the 40 weeks therefore fall before conception, which is why 26 weeks of gestational age is not 26 weeks since conception.
What does a negative days remaining mean?
It means the count date is after the estimated due date. The page reports how many days have passed and keeps counting gestational age beyond 40 weeks instead of hiding the result.
Can I use this for a multiple pregnancy?
The page counts calendar days from a single due date. It does not adjust for twins, triplets or any condition that changes the planned delivery date, so use the date your care team gives you.