When Did I Conceive if My Due Date Is December 11?
If your baby is due on December 11, conception most likely happened around March 20. Here is how that date is worked out and the range of days it could realistically fall on.
Key dates for a December 11 due date
| Most likely conception date | March 20 |
|---|---|
| Likely conception window | March 17 – March 23 |
| Days when sex could have led to pregnancy | March 15 – March 21 |
| Estimated first day of last period | March 6 |
| Positive home test likely from | April 3 (around the missed period) |
| Second trimester begins (14 weeks) | June 12 |
| Third trimester begins (28 weeks) | September 18 |
| Full term begins (39 weeks) | December 4 |
How the conception date is calculated
Due dates are normally counted as 280 days (40 weeks) from the first day of the last menstrual period. In a typical 28-day cycle, ovulation and conception happen about two weeks after that period starts, so conception sits roughly 266 days before the due date. Subtracting 266 days from December 11 gives March 20.
Conception happens when sperm meets the egg, usually within a day of ovulation. Since sperm may wait up to five days for an egg to be released, the window for sex that could have caused the pregnancy is wider than a single day.
How certain is this date?
Only a small share of babies, often quoted at around 4 to 5 percent, arrive on their exact due date, so dates worked backwards from a due date carry a margin of a few days either way.
If your cycles are usually longer than 28 days, conception probably happened a little later than March 20; with shorter cycles, a little earlier. A cycle that is 32 days long, for example, shifts the likely date about four days later.
Check your own dates
Enter your exact due date in the conception date calculator to see these results with weekdays and the correct year, or start from your last period with the last-period conception calculator.
Conception Date Calculator: Find the most likely conception date from your due date.
Open the calculatorThis article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.