When Did I Conceive if My Due Date Is September 12?
Counting back from a September 12 due date puts the most likely day of conception at December 20 of the previous year. That's an estimate, so we've also included the window of days around it.
Key dates for a September 12 due date
| Most likely conception date | December 20 |
|---|---|
| Likely conception window | December 17 – December 23 |
| Days when sex could have led to pregnancy | December 15 – December 21 |
| Estimated first day of last period | December 6 |
| Positive home test likely from | January 3 (around the missed period) |
| Second trimester begins (14 weeks) | March 14 |
| Third trimester begins (28 weeks) | June 20 |
| Full term begins (39 weeks) | September 5 |
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 September 12 gives December 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?
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 December 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.