Pregnancy Due Date (EDD) Calculator
Enter the first day of your last period (LMP) or your date of conception to instantly calculate your Estimated Due Date (EDD). Watch your progress visually across all three trimesters.
Most women have a 28-day cycle. Adjust this if yours is typically shorter or longer.
Select your dates to calculate your estimated due date, current trimester, and pregnancy timeline.
How the due date is estimated
Naegele's rule adds 280 days -- forty weeks -- to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14, which is why the calculator adjusts for a cycle that is longer or shorter than that.
A 32-day cycle pushes the estimate four days later, because ovulation happened four days later than the rule assumes. A 25-day cycle pulls it three days earlier. Entering your real cycle length matters more than most people expect.
| Cycle length | Adjustment | Estimated due date shifts |
|---|---|---|
| 25 days | -3 days | Three days earlier |
| 28 days | None | Standard 280 days |
| 30 days | +2 days | Two days later |
| 35 days | +7 days | One week later |
Counting from conception instead
Pregnancy is dated from the last period, not from conception, so a pregnancy is counted as roughly two weeks along at the moment of conception. If you know the conception date, the calculator subtracts fourteen days to reach an equivalent LMP and then proceeds identically.
That means the gestational age shown will always be about two weeks more than the actual age of the embryo. This is the convention used by every maternity service, so it is what your notes and scans will use too.
Trimesters and what the weeks mean
The trimester boundaries used here follow the common clinical convention, counted in completed weeks from the LMP.
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 0 - 13 | Up to about three months |
| Second | 14 - 26 | Three to six months |
| Third | 27 onwards | Six months to birth |
- A term birth is anything from 37 to 42 weeks, not the due date itself.
- A first-trimester dating scan can move the estimate by a week or more, and the scan date takes precedence.
- Book antenatal care through your local MOH clinic or a private obstetrician early rather than waiting for the estimate to firm up.
Why an early scan beats the calendar
Calendar dating assumes ovulation happened a predictable number of days into the cycle, which is a reasonable assumption for a regular 28-day cycle and a poor one otherwise. Cycles that run long, short, or irregular shift ovulation with them, and an uncertain last-period date compounds the problem.
An early ultrasound measures the fetus directly rather than inferring from the calendar, which is why a dating scan in the first trimester is generally more accurate than any calculation from a last menstrual period. Where the two disagree by more than a set margin, clinicians normally revise the due date to the scan.
None of which makes calendar dating useless: it is what establishes roughly where you are before a scan is arranged, and it is what you will be asked for at a first appointment. Treat it as the starting estimate, and expect your clinician to refine it.