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Estimate the due date
On the default — last period January 20, 2026, 28-day cycles — the estimated due date is Tuesday, October 27, 2026.
FREE TOOL · HEALTH
Pregnancy Due Date Calculator
Enter the first day of your last menstrual period and your usual cycle length. You get the estimated due date from Naegele’s rule, the likely conception window, trimester dates, how far along you are today, and the countdown.
Planning estimates only — not medical advice. This calculator applies the standard obstetric formulas for planning and curiosity. Confirm every date with your healthcare provider; a first-trimester ultrasound is the most accurate way to date a pregnancy.
Naegele’s rule: the 280-day method
Doctors date a pregnancy from the first day of the last menstrual period, not from conception — because the period is a date you know, and conception usually isn’t. Naegele’s rule: add one year, subtract three months, add seven days to that date. The shortcut version the calculator uses: LMP + 280 days (40 weeks), adjusted by (your cycle length − 28) days for longer or shorter cycles. It is named for the German obstetrician Franz Naegele, who published it around 1830 — and it still beats most people’s guesses.
The key dates from your last period
For the default — last period Tuesday, January 20, 2026, 28-day cycles. Change the inputs and the calculator rebuilds this timeline for you.
| Milestone | Date | Weeks |
|---|---|---|
| Likely conception (ovulation) | Tue, Feb 3, 2026 | 2w 0d |
| End of first trimester | Mon, Apr 27, 2026 | 13w 6d |
| 20-week anatomy scan | Tue, Jun 9, 2026 | 20w 0d |
| Viability milestone | Tue, Jul 7, 2026 | 24w 0d |
| Start of third trimester | Tue, Aug 4, 2026 | 28w 0d |
| Full term | Tue, Oct 6, 2026 | 37w 0d |
| Estimated due date | Tue, Oct 27, 2026 | 40w 0d |
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Estimated due date | LMP + 280 days + (cycle − 28) | Tuesday, October 27, 2026 — the 40-week mark, not a delivery appointment. |
| Conception estimate | LMP + (cycle − 14) days | Around Tuesday, February 3, 2026 — ovulation day in a textbook cycle. |
| Weeks pregnant | (today − LMP) ÷ 7 | 39 weeks, 2 days on October 22, 2026. |
| Days to go | due date − today | 5 days — the home stretch. |
| Trimester | weeks 1–13 / 14–27 / 28–40 | Third trimester since August 4, 2026. |
Why the due date moves
Naegele’s rule assumes ovulation on day 14, but real ovulation varies — so a first-trimester ultrasound (crown-rump length, accurate to about ±5–7 days) often revises the estimate, and the ultrasound date wins. Irregular cycles, late ovulation, and IVF (dated from egg retrieval or transfer) all shift it too. And remember: only about 5% of babies arrive on the estimated date — most come sometime between 37 and 42 weeks.
Who uses this
- Expecting parents: the first number everyone asks for — and the anchor for every plan after it.
- Partners & family: travel, time off, and “when should we visit?” all start here.
- Doulas & midwives: intake timelines and on-call scheduling.
- HR & benefits teams: parental-leave timelines begin with the due date.
Accuracy & limitations
Naegele’s rule is an estimate built on averages: 28-day cycles, ovulation on day 14, 280-day gestation. Real pregnancies vary on all three. The calculator adjusts for cycle length but cannot know your actual ovulation day — only an ultrasound measures the pregnancy directly. Treat every output as a planning estimate and confirm it with your healthcare provider.
Counting down to the big day? The countdown calculator ticks toward any date, and the sleep cycle calculator helps protect the sleep you’ll soon miss. For everyday body math, the TDEE calculator covers daily calorie needs.
Cite this calculatorBright Side Kit. “Pregnancy Due Date Calculator.” brightoolbox.com, 2026.
Bright Side Kit. "Pregnancy Due Date Calculator." https://brightoolbox.com/pregnancy-due-date-calculator/ Accessed October 22, 2026.Frequently asked questions
What is Naegele’s rule?
Around 1830, Franz Naegele proposed estimating the due date as last period + 1 year − 3 months + 7 days — equivalently, LMP + 280 days. It remains the standard starting point worldwide.
Why is the due date counted from my last period instead of conception?
Because the period is a date you actually know. Conception happens around ovulation, roughly two weeks after the period in a textbook cycle — which is why “weeks pregnant” runs about two weeks ahead of the embryo’s true age.
What if my cycles are irregular or longer than 28 days?
Enter your average cycle length and the calculator shifts the estimate by (cycle − 28) days. With very irregular cycles, though, a first-trimester ultrasound is far more reliable — say so at your first appointment.
What if I know the exact conception date?
Add 266 days (38 weeks) to it — gestation from conception is about two weeks shorter than from the last period. IVF pregnancies are dated from egg retrieval or embryo transfer instead.
Will an ultrasound change my due date?
Often, yes — and the ultrasound date wins. First-trimester crown-rump-length measurement dates a pregnancy to about ±5–7 days, which beats calendar math whenever they disagree.
How accurate is the estimated due date?
It is a midpoint, not an appointment: only about 5% of babies arrive on the estimated date. Roughly 80% arrive within two weeks of it, between 37 and 42 weeks.
Does IVF use a different calculation?
Yes — IVF dates from a known event: egg retrieval + 266 days, or transfer day minus the embryo’s age (3 or 5 days) + 266 days. Naegele’s rule doesn’t apply.