Due date from conception: the 266-day rule
From conception to birth, a typical pregnancy lasts about 266 days, or 38 weeks. The familiar “40 weeks” figure includes the two weeks between the start of the last period and ovulation. So when the conception date is known, the due date is simply:
Due date = conception date + 266 days
Example: conception on April 10 gives an estimated due date of January 1 the following year. Your gestational age (“weeks pregnant”) is still counted from an equivalent period date 14 days before conception, so on the day of conception you are counted as 2 weeks pregnant.
Which date should I use?
- Ovulation tests or temperature charting: use the day of ovulation — it is the best stand-in for the conception date.
- IUI (intrauterine insemination): use the insemination date.
- A single intercourse date: you can use it, but remember conception may have happened up to a few days later.
- IVF: use the IVF Due Date Calculator instead, which accounts for the embryo’s age.
Example due dates from conception
| Conception date | Estimated due date (+266 days) |
|---|---|
| January 15, 2027 | October 8, 2027 |
| March 1, 2027 | November 22, 2027 |
| June 20, 2027 | March 12, 2028 |
| September 5, 2027 | May 28, 2028 |
Notice that the last two due dates fall after February 29, 2028 — the calculator counts the leap day automatically. For a quick overview by month, see the guides for a conception in January, June or September, which list the due date for every day of the month.
Due date based on conception vs. last period
A due date calculator based on conception and one based on the last menstrual period (LMP) give the same answer when ovulation happens on day 14 of a 28-day cycle, because conception is then exactly 14 days after the LMP. When they differ, the conception method is usually closer, because it does not rely on assumptions about when you ovulated:
| From conception | From last period | |
|---|---|---|
| Days added | 266 | 280 (± cycle adjustment) |
| Main assumption | The date entered is the day of ovulation / fertilisation | Ovulation about 14 days before the next period |
| Best for | Ovulation tracking, IUI, a single possible date | Regular cycles and a known period date |
| Main source of error | Using the intercourse date when conception came days later | Irregular cycles or an uncertain period date |
Whichever you use, your care team will normally confirm the due date with an early ultrasound. The main Due Date Calculator lets you switch between both methods to compare.
Planning ahead
You can also enter a future date to see when a baby conceived then would be due — handy if you are trying to conceive and want to know which months lead to which birth months. For the reverse question — when did conception happen? — see the Conception Date Calculator. The full explanation of every method is on the main Due Date Calculator page.