When Did I Conceive if My Due Date Is December 17?
Counting back from a December 17 due date puts the most likely day of conception at March 26. That's an estimate, so we've also included the window of days around it.
Key dates for a December 17 due date
| Most likely conception date | March 26 |
|---|---|
| Likely conception window | March 23 – March 29 |
| Days when sex could have led to pregnancy | March 21 – March 27 |
| Estimated first day of last period | March 12 |
| Positive home test likely from | April 9 (around the missed period) |
| Second trimester begins (14 weeks) | June 18 |
| Third trimester begins (28 weeks) | September 24 |
| Full term begins (39 weeks) | December 10 |
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 17 gives March 26.
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?
Due dates set from an early ultrasound are usually more accurate than those set from a period date alone, so if your provider changed your due date after a scan, use the newer one here.
If your cycles are usually longer than 28 days, conception probably happened a little later than March 26; 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.