Pregnancy Due Date Calculator
Due date, conception estimate, and current week from the first day of the last period.
What it does
Pregnancy is dated from the first day of the last period, not from conception, which adds about two weeks before ovulation even happens. The standard Naegele rule adds 280 days to that date, shifting for longer or shorter cycles. Full term centers on week 40, with weeks 37 to 42 counted as the normal window.
The tool above applies that rule to your dates. Enter the last period start and your usual cycle length to get the due date, the likely conception day, and the current week with trimester.
Calendar math is a starting point. First trimester ultrasound measures the embryo directly and dates more exactly, so the scan date replaces the calendar date whenever they differ.
How to use
- Enter the first day of the last menstrual period, not spotting.
- Enter your usual cycle length in days.
- Read the due date and the conception estimate together.
- Use the progress line for the current week, and confirm everything at the first scan.
Formula
- 280 days
- 40 weeks of gestation by convention.
- cycle - 28
- Shift for longer or shorter cycles.
- +14 days
- Ovulation offset for the conception estimate.
The 280 days equal 40 weeks of gestation by convention. Longer cycles push ovulation later, so each extra cycle day shifts both answers one day forward. Irregular cycles make this rough, and the dating scan takes over.
Worked example
Example: LMP 1 Jan 2026, 28 day cycle
Last period starts 1 January 2026, regular 28 day cycles.
- Add 7 days: 1 Jan becomes 8 Jan (quick check form of the rule).
- Full math: 1 Jan + 280 days = 8 Oct 2026.
- Cycle shift: 28 - 28 = 0, so no shift.
- Due date: 8 October 2026.
- Conception estimate: 1 Jan + 14 days = 15 January 2026.
Reference table
| Marker | When | Note |
|---|---|---|
| Conception | About 2 weeks after LMP | Ovulation window in a 28 day cycle |
| First trimester ends | Week 13 | Dating scan usually by week 12 |
| Second trimester ends | Week 27 | Anatomy scan near week 20 |
| Early term | Weeks 37 to 38 | Normal but worth noting |
| Full term | Weeks 39 to 40 | Due date sits at 40 weeks |
Limits
- Regular cycles assumed. Irregular ones need early scans.
- Spotting is not the period start.
- Few babies arrive on the exact date.
- Late revisions follow first trimester scans.
Common mistakes
- Dating from conception or a positive test instead of the last period.
- Using one unusual cycle length instead of the usual average.
- Treating the due date as an appointment. Only a small share arrive that day.
- Ignoring the dating scan when it disagrees with calendar math.
- Counting spotting as the period start.
Try it yourself
Quick checks. Answers sit tucked under each question.
LMP 1 Mar 2026, 30 day cycle. Due date?
1 Mar + 280 days = 6 Dec, plus 2 cycle days = 8 Dec 2026, a Tuesday.
Conception estimate for the same dates?
1 Mar + 14 + 2 = 17 March 2026.
Questions
How accurate is the due date?
Within about two weeks for regular cycles with known LMP. First trimester ultrasound narrows it to days.
Why count from the period, not conception?
Ovulation day is rarely known exactly, while the period start is a clear date. The 40 week convention absorbs the offset.
What if my cycles are irregular?
Calendar math turns rough. Book an early dating scan, which measures instead of guessing.
Can the due date change later?
Yes. Scans can revise it, mainly in the first trimester when measurement is most exact.
What does full term mean?
39 to 40 weeks. Weeks 37 to 38 count as early term and 41 to 42 as late term.