How a due date is calculated from conception
A due date is set at 40 weeks — 280 days — after the first day of the last period. In a 28-day cycle, ovulation and conception happen around day 14, which leaves 266 days (38 weeks) between conception and the due date. That is the calculation used here:
Estimated due date = conception date + 266 days
Example: a conception date of May 20, 2026 gives an estimated due date of February 10, 2027. On the same dates, the pregnancy is counted as having started on May 6, 2026 — the equivalent “last period” date that every week and trimester is measured from.
When this method works best
- You tracked ovulation with tests, temperature or a fertility monitor.
- There was only one possible date of conception.
- You had intrauterine insemination (IUI) — use the insemination date.
- Your cycles are irregular, so an LMP-based date would be less reliable.
For IVF, use the IVF Due Date Calculator instead, because the embryo’s age at transfer changes the calculation. For every method in one place, see the main Due Date Calculator.
How accurate is a due date from conception?
If the ovulation date is right, a conception-based due date is at least as good as an LMP-based one, because it does not rely on the assumption that ovulation happened on day 14. Its weakness is that many people only know when they had intercourse, not when conception happened. Either way, your provider may still confirm or adjust the date after a dating ultrasound.