Estimate your due date from a known conception date.
Due date = conception date + 266 days, the average gestation length measured from ovulation.
This calculator provides general informational estimates and is not a medical diagnosis. Results should be interpreted alongside other factors. If you have concerns about your health, consider speaking with a qualified healthcare professional.
The most common method for estimating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is the standard used by most healthcare providers as a starting estimate, later refined with ultrasound measurements if needed.
An equivalent shortcut: subtract 3 months from the LMP date, then add 7 days and 1 year. Both methods produce the same 40-week estimate, assuming a standard 28-day cycle.
| Method | Basis | Typical accuracy |
|---|---|---|
| LMP (Naegele's Rule) | Last menstrual period | ±2 weeks |
| Conception date | Known ovulation/conception | ±1–2 weeks |
| First-trimester ultrasound | Fetal crown-rump length | ±3–5 days (most accurate) |
| IVF transfer date | Known embryo age at transfer | Very precise |
Example 1 — From conception date: A known conception date of 1 December 2025 gives an estimated due date of 1 December 2025 + 266 days ≈ 23 August 2026.
Example 2 — From LMP (Naegele's Rule): A last menstrual period starting 20 November 2025 gives a due date of 20 November 2025 + 280 days ≈ 27 August 2026 — about 14 days later than the conception-based estimate, reflecting the roughly two-week gap between LMP and actual ovulation in a standard cycle.
Example 3 — Adjusted cycle length: For someone with a 35-day cycle instead of the standard 28-day cycle, ovulation typically occurs about a week later, so providers often add roughly 7 extra days to the standard LMP-based estimate.
| Trimester | Weeks | Typical milestone |
|---|---|---|
| First | 1–13 | Dating ultrasound, early screening |
| Second | 14–27 | Anatomy scan, gender reveal possible |
| Third | 28–40+ | Growth monitoring, birth planning |
How many babies are actually born on their due date? Only a small percentage — most births occur within a week or two before or after the estimated date.
Is ultrasound dating more accurate than LMP dating? Yes, especially in the first trimester, since it directly measures fetal size rather than relying on cycle assumptions.
Why did my due date change after an ultrasound? Providers often adjust the estimate if the ultrasound measurement differs meaningfully from the LMP-based calculation.
What counts as "full term"? Full term is generally defined as 39 weeks 0 days through 40 weeks 6 days; "early term" (37–38 weeks) and "late term" (41 weeks) are also considered within a normal range.
Why is the due date based on 280 days from LMP instead of 266 days from conception? Because conception date is often unknown, LMP is used as a practical, consistently trackable reference point; the extra 14 days accounts for the average time from LMP to actual ovulation.
Does an irregular cycle make the due date less reliable? Yes — LMP-based dating assumes a fairly standard 28-day cycle, so irregular cycles make an early ultrasound more useful for an accurate estimate.
Can the due date be recalculated later in pregnancy? It's typically set early (first trimester) and generally not changed later unless there's a significant discrepancy, since later ultrasounds are less precise for dating due to variable fetal growth rates.
A conception date of October 10th projects an estimated due date around July 3rd of the following year, roughly 266 days later.
Full term: A pregnancy that has reached the standard delivery window, generally 37–42 weeks.
Gestational age: Pregnancy age counted from the last menstrual period.