What the due date calculator does
Enter the first day of your last menstrual period (LMP) and the calculator returns the estimated due date — the day you reach 40 weeks of pregnancy. It uses the same rule clinicians use for an initial estimate, so the date should match what a doctor or midwife writes down at the first appointment before an ultrasound refines it. The calculation runs in your browser; the date you enter is not stored or sent anywhere.
Naegele's rule
Due date = first day of LMP + 280 days (40 weeks)
Equivalent shortcut: LMP + 1 year − 3 months + 7 days
LMP 10 March 2026 → 10 March 2027 − 3 months = 10 December 2026 + 7 days = 17 December 2026The rule dates from the early 1800s and assumes a 28-day cycle with ovulation on day 14, so the 280 days include roughly two weeks before conception actually happened. That is why pregnancy is counted as 40 weeks even though the baby has been developing for about 38.
How accurate is it?
Only about 4–5% of babies arrive on the estimated date. Most are born within a two-week window either side: roughly 80% between 38 and 42 weeks. The estimate is a planning reference, not a prediction. Sources of error:
- Cycle length. With a 35-day cycle, ovulation is around day 21, so the LMP-based date is about a week too early. Add (cycle length − 28) days to correct.
- Uncertain LMP. Irregular periods, recent contraceptive use or breakthrough bleeding make the LMP unreliable.
- Early ultrasound between 8 and 13 weeks measures the embryo directly and is accurate to about ±5 days. If it differs from the LMP date by more than a week, clinicians usually adopt the scan date.
- First pregnancies tend to run slightly longer on average — median around 40 weeks 5 days in some studies.
Key milestones from the LMP
| Week | Milestone |
|---|---|
| 4 | Missed period; home test usually positive |
| 6–8 | Heartbeat visible on ultrasound; first appointment in many countries |
| 10–13 | Dating scan and first-trimester screening |
| 12–14 | End of first trimester |
| 18–22 | Anatomy (anomaly) scan |
| 24–28 | Glucose screening; viability threshold around 24 weeks |
| 27–28 | Third trimester begins |
| 36 | Group B strep test (in some countries); baby usually head-down |
| 37 | Early term — baby considered full term from 39 weeks |
| 40 | Estimated due date |
| 41–42 | Induction commonly offered if labour has not started |
Other ways to date a pregnancy
- Conception date known (IVF transfer, tracked ovulation): due date = conception + 266 days. For IVF, count from egg retrieval or transfer as the clinic instructs.
- Ultrasound crown–rump length in the first trimester is the most accurate method and overrides the LMP date when they disagree by more than about seven days.
- Fundal height (tape measurement of the uterus) is a rough check later in pregnancy, not a dating method.