Due Date Calculator · Practical guide
Due Date Calculator by Conception Date
Know when you conceived? Count forward 266 days — here is the math and how to run it through the calculator above.
At a glance
The Conception-Date Formula
If you know when conception occurred — from an ovulation test, cycle tracking, or IVF — the due date is conception date + 266 days. That is 38 weeks, the average time from fertilization to birth. It agrees with the standard last-period method because conception in a 28-day cycle happens about 14 days after the LMP: 280 − 14 = 266.
Interactive tool
Due Date Calculator
Enter your own values, then compare the result with the examples below.
Trimester Dates
| Trimester | Start Date | End Date |
|---|---|---|
| First Trimester | -- | -- |
| Second Trimester | -- | -- |
| Third Trimester | -- | -- |
Worked Example
Suppose conception occurred around January 24, 2026. Adding 266 days lands on October 17, 2026 — the same date as counting 280 days from a January 10 last period. The two methods are two views of the same pregnancy.
Using the Calculator Above with a Conception Date
This calculator starts from the last menstrual period, but the conversion is easy: subtract 14 days from your conception date and enter the result as your LMP with a 28-day cycle. Conceived June 20? Enter June 6 as the last period, and the due date, trimester table, and fetal age all line up correctly.
Quick Reference
| Conception Date | Enter as LMP | Estimated Due Date |
|---|---|---|
| January 24, 2026 | January 10, 2026 | October 17, 2026 |
| March 15, 2026 | March 1, 2026 | December 6, 2026 |
| May 15, 2026 | May 1, 2026 | February 5, 2027 |
A Note on Precision
Even a known conception date has wiggle room: sperm can survive up to five days, so intercourse on one day can lead to fertilization several days later. Treat any due date as the middle of a two-week window — only about 4-5% of babies arrive on the exact day.
Frequently Asked Questions
Why 266 days from conception instead of 280?
The 280-day rule counts from the last period, which starts about two weeks before ovulation and conception. Subtracting those 14 days gives 266 days — 38 weeks — from conception to the due date.
Is a due date from conception more accurate than from LMP?
If the conception date is genuinely known — from IVF or a confirmed ovulation test — yes, it removes the guesswork about cycle length and ovulation timing that the LMP method assumes.
What if I only know the week I conceived?
Pick the middle of that week, add 266 days, and treat the result as plus or minus about a week. A first-trimester ultrasound can narrow it down to within 5-7 days.
Does IVF change the calculation?
IVF due dates are calculated from the transfer date and embryo age — for a day-5 embryo, the due date is 261 days after transfer. Your clinic's date takes priority over any general calculator.