An estimated due date supports pregnancy dating; it is not a guaranteed birth date
A common convention estimates the due date as 280 days, or 40 weeks, from the first day of the last menstrual period. This provides a reference point for prenatal care; it does not mean birth will occur on that exact day. Birth can happen before or after it, and clinical care determines appropriate monitoring and follow-up.
The American College of Obstetricians and Gynecologists (ACOG) recommends determining, discussing, and documenting gestational age and the estimated due date as soon as last-menstrual-period data, the first accurate ultrasound, or both are available. A calculator can explain an initial estimate, but it does not replace the pregnancy dating recorded in clinical care.
The formula used by Calcora
LMP means the first day of the last menstrual period, not its final day. The model adds 280 days when average cycle length is 28 days. If the entered average differs, it adds or subtracts the difference as a simplified adjustment for assumed ovulation timing.
This adjustment does not measure actual ovulation. Cycle regularity and the accuracy of the remembered average matter, and one unusual cycle should not automatically replace the person's usual average.
A complete date and cycle-length example
If the first day of the last period was 1 January 2026 and average cycle length was 28 days:
The estimated date is 8 October 2026. With a 32-day average cycle and the same starting date, the model adds four days, producing 12 October 2026. This is a changed model input, not an automatic medical revision of a clinician-documented due date.
| First day of last period | Average cycle | Model adjustment | Calculated estimate |
|---|---|---|---|
| 1 January 2026 | 28 days | 0 days | 8 October 2026 |
| 1 January 2026 | 32 days | +4 days | 12 October 2026 |
| 1 January 2026 | 26 days | −2 days | 6 October 2026 |
How gestational age appears in weeks and days
Calendar-based gestational age counts days elapsed from the first day of the last period to the reference date. Completed weeks are the whole-number result of division by seven; the remainder is additional days. In the previous example, 1 January to 9 April 2026 is 98 days, or 14 weeks and 0 days.
Additional days = Gestational days mod 7
100 days = 14 weeks + 2 days
The phrase “week 14” can be used differently from “14 completed weeks” in some contexts. The calculator therefore shows completed weeks and additional days instead of relying on an ambiguous verbal label.
Why conception dating differs from last-period dating
Conventional gestational dating includes roughly two weeks before conception in the usual model, so counting does not start from conception in the same way. In the linked conception calculator, 14 days are added to the difference between a known conception date and the assessment date, while the 40-week estimate is conception date plus 266 days.
For example, a known conception date of 15 January 2026 gives 8 October 2026 when 266 days are added. Do not infer an unknown conception date from a calculator result. Embryo transfer and assisted reproduction require procedure-specific dates and the appropriate clinical tool and guidance.
Trimesters, ultrasound, and later revisions
The site's trimester calculator uses a simplified calendar grouping: below 14 weeks for the first, 14 to below 28 for the second, and 28 onward for the third. This describes the entered week, not maternal or fetal health.
ACOG explains that last-period data and accurate early ultrasound are used for dating. It considers pregnancy without an ultrasound confirming or revising the estimate before 22 weeks to be suboptimally dated. It also recommends that subsequent due-date changes be rare, discussed with the patient, and documented. In practice, do not revise a clinical due date whenever a home calculator gives a different result; discuss the difference with the care team.
Read the other outputs within their limits
- Ovulation date and fertile window: model estimates based on cycle length, not an ovulation test or contraception method.
- Next expected period: a calendar addition using entered cycle length, not confirmation that it will occur.
- Weeks and trimester: a conversion of dates and duration, not a direct measurement of fetal development.
- Beyond the estimated date: does not alone establish a problem or a particular intervention; clinical follow-up interprets the situation.
Frequently asked questions
Should I enter the last day of my period?
No. Enter the first day of the last menstrual period. Entering its final day shifts every estimated date.
Does 40 weeks mean birth will happen on that exact day?
No. The date is an estimated reference point, and birth can occur before or after it. Do not judge the pregnancy's course from that date alone.
Should I rely on the last period or ultrasound?
A calculator cannot make that clinical decision. The care team determines the best obstetric estimate from cycle dates, early ultrasound, and available information, and documents any revision.
Why did the estimate change when I changed cycle length?
The model adds the difference between entered cycle length and 28 days to the 280-day estimate. This is a simplified assumption, not a measurement of actual ovulation.
Sources and references
- ACOG — When Pregnancy Goes Past Your Due Date: the 280-day or 40-week convention from the first day of the last period and the estimated nature of the date. Accessed 2026-11-07.
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date: last-period and early-ultrasound data, documentation, and limits on later revision.
- All example dates are hypothetical and calculated with the Gregorian calendar. This is general education, not diagnosis, prenatal planning, or personal medical advice.