Due Date Calculator
Estimate pregnancy due date based on last menstrual period.
What Is the Due Date Calculator?
A due date calculator projects your baby's expected arrival from the first day of your last menstrual period or conception date, applying standard obstetric conventions instantly. The classic method, called Naegele's rule, adds 280 days to the last period start, assuming a 28 day cycle with ovulation around day 14. Providers confirm and adjust these estimates with first trimester ultrasound, which is the most accurate dating method. Remember the word estimate: full term spans a five week window, and only a small minority of babies arrive exactly on their predicted day. Use the date for planning leave, showers, and childcare logistics, not as a countdown that ends in disappointment.
Key Statistics
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280 days
Standard gestation length added to the last period start under Naegele's rule
Source: Standard obstetric practice
-
About 4%
Share of babies born exactly on their estimated due date
Source: Birth timing research
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39 to 40 weeks
Range considered full term by current obstetric guidelines
The Formula
Due Date = LMP + 280 days (40 weeks).
Worked Examples
If your last period started on March 15, 2026, and you have a typical 28-day cycle, your estimated due date would be around December 20, 2026. Only about 5% of babies arrive exactly on their due date.
Dating from the last period
- Last menstrual period began March 10, 2026
- Add 280 days to that date
- Count forward month by month to December
Estimated due date is December 15, 2026.
Adjusting for a longer cycle
- Period start March 10 with a consistent 32 day cycle
- Ovulation likely occurred about 4 days later than the standard assumption
- Shift the standard due date later by those days
Adjusted estimate moves to about December 19, 2026, pending ultrasound confirmation.
Real World Use Cases
Early pregnancy orientation
Get a working timeline immediately while awaiting the confirming appointment.
Leave and benefits paperwork
Plan parental leave windows and notify employers with realistic dates.
Family logistics
Coordinate travel, nursery setup, and support visits around the estimate window.
IVF and known conception dates
Calculate precisely from transfer dates, which produce the most predictable timelines.
Expert Tips
- ✓ First trimester ultrasound dating overrides period math whenever the two disagree meaningfully.
- ✓ Longer cycles typically mean later ovulation, so extend the estimate accordingly.
- ✓ Plan events for the second trimester sweet spot between weeks 14 and 30 when energy is higher.
- ✓ Only about one in twenty five babies arrives on the exact date; think in terms of a due month.
- ✓ Note the difference between estimated date of delivery conventions across countries; some count 40 weeks from period start, others from conception.
Frequently Asked Questions
How is a due date calculated? ▼
The classic method adds 280 days (40 weeks) to the first day of your last menstrual period, called Naegele rule. It assumes a 28 day cycle with ovulation around day 14.
How accurate is the due date? ▼
A due date is an estimate, not a promise. Only about 4% to 5% of babies arrive exactly on the date, and a full term pregnancy can span 37 to 42 weeks. An early ultrasound is the most accurate way to date a pregnancy.
What if I do not know my last period date? ▼
If you cannot remember your last menstrual period or your cycles are irregular, an ultrasound in the first trimester is the best way to establish the due date. It measures the baby to estimate gestational age.
How is an IVF due date calculated? ▼
For IVF, the due date is based on the embryo transfer date, not the last period. The calculator counts from transfer day using the embryo age (day 3 or day 5) to set the pregnancy timeline.
What is gestational age? ▼
Gestational age is how far along a pregnancy is, counted in weeks from the first day of the last menstrual period. A full term pregnancy is about 40 weeks of gestational age.
Why is my ultrasound due date different from my period due date? ▼
The two dates differ when ovulation happened later or earlier than day 14 of a 28 day cycle, or when cycle length varies. Providers usually trust the first trimester ultrasound estimate.
How is a due date calculated? ▼
Add 280 days to the first day of the last menstrual period, or add 266 days to the estimated conception date, then refine with ultrasound.
How accurate are due dates? ▼
Most babies arrive within two weeks either side of the estimate, with first time mothers averaging slightly longer pregnancies.
Common Mistakes to Avoid
- ⚠ Treating the due date as an exact date rather than a range. Only about 5% of babies arrive on their exact due date. A full term pregnancy spans 37 to 42 weeks, so being "overdue" by a week is still within the normal range.
- ⚠ Using a 28-day cycle when your cycle is actually longer or shorter. For a 35-day cycle, ovulation occurs around day 21 instead of day 14, shifting the due date about 7 days later. Enter your actual average cycle length for a personalized estimate.
- ⚠ Confusing the LMP-based due date with conception date. A due date of December 20 based on a March 15 LMP does not mean conception happened in December. Conception occurred roughly 2 weeks after the LMP (around March 29), roughly 38 weeks before the due date.
- ⚠ Confusing the last period start with conception. They differ by roughly two weeks in a standard cycle, which shifts estimates materially.
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