Due Date by Conception Calculator
Estimate pregnancy due date from conception date.
A due date by conception calculator estimates a baby's expected delivery date based on a known or estimated conception date, using the standard clinical assumption of a 266-day (38-week) gestation period from conception to birth. This differs from the more commonly used last-menstrual-period (LMP) method, which instead counts 280 days (40 weeks) from the first day of the last period, since ovulation and conception typically occur about two weeks after the LMP begins.
Enter your known or estimated conception date to calculate an estimated due date.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| Conception date: March 1 | Estimated due date: November 22 | Adding 266 days to March 1 lands on approximately November 22 of the same year. |
| Conception date: June 10 | Estimated due date: March 3 (following year) | Adding 266 days to June 10 crosses into the next calendar year, landing on approximately March 3. |
Why Conception-Based and LMP-Based Due Dates Differ by About Two Weeks
The standard 280-day (40-week) due date calculation counts from the first day of the last menstrual period, not from conception itself, because most people know their LMP date more reliably than their exact conception date. Since ovulation and conception typically occur roughly 14 days after the LMP begins in an average cycle, a conception-based estimate (266 days from conception) and an LMP-based estimate (280 days from LMP) should generally land on a very similar due date when both dates are accurately known.
When Conception-Based Dating Is More Useful
For people who know their exact conception date - for example, through fertility tracking, a single intercourse date, or IVF/fertility treatment records - a conception-based calculation can be more precise than an LMP-based estimate, particularly for those with irregular menstrual cycles where the standard 14-day-post-LMP ovulation assumption may not hold true.
Due Dates Are Estimates, Not Predictions
Only a small percentage of babies are actually born on their calculated due date - most births occur within a window of about two weeks before or after it. A due date is best understood as the estimated midpoint of a normal delivery window rather than a precise prediction, which is why healthcare providers typically discuss a range rather than treating the exact date as a guarantee.
How to Use the Due Date by Conception Calculator
- Open the Due Date by Conception Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the due date by conception calculator works.
Frequently Asked Questions
How do I calculate my due date if I know my exact conception date?
Add 266 days (38 weeks) to your conception date. This calculator performs that calculation automatically and accounts for calendar month-length variation.
Why is the due date calculated differently from conception versus last period?
Due dates from the last menstrual period (LMP) count 280 days (40 weeks) because ovulation typically occurs about 14 days after the LMP begins. Counting 266 days from conception itself lands on essentially the same estimated date when both dates are accurate.
Is conception-date dating more accurate than LMP dating?
It can be more precise for people who know their exact conception date (through fertility tracking or treatment records), especially those with irregular cycles where the standard LMP-to-ovulation assumption may not apply as reliably.
How likely is a baby to be born exactly on the due date?
Not very likely - only a small percentage of babies are born on their exact calculated due date. Most births occur within about two weeks before or after it, so a due date is best treated as an estimated midpoint rather than a precise prediction.
Can my doctor adjust my due date?
Yes. Healthcare providers often refine the estimated due date using ultrasound measurements taken during pregnancy, which can be more precise than date-based calculations alone, especially if the exact conception or LMP date is uncertain.
What if I don't know my exact conception date?
If your conception date is uncertain, a due date calculator based on your last menstrual period, or an ultrasound-based estimate from your healthcare provider, will generally provide a more reliable estimate.
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Last updated: September 27, 2026.