Due Date September 13: When Did You Conceive?
Wondering when your September 13 baby was conceived? Using the standard 266-day rule, the answer is most likely December 21 of the previous year, give or take a few days.
Key dates for a September 13 due date
| Most likely conception date | December 21 |
|---|---|
| Likely conception window | December 18 – December 24 |
| Days when sex could have led to pregnancy | December 16 – December 22 |
| Estimated first day of last period | December 7 |
| Positive home test likely from | January 4 (around the missed period) |
| Second trimester begins (14 weeks) | March 15 |
| Third trimester begins (28 weeks) | June 21 |
| Full term begins (39 weeks) | September 6 |
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 13 gives December 21.
Sperm can survive in the reproductive tract for up to five days, while an egg lives for about 12 to 24 hours after ovulation. That means sex several days before ovulation can still lead to pregnancy.
How certain is this date?
Remember that a due date is only the midpoint of a range. Ovulation timing varies from cycle to cycle, so the true conception day can sit a few days away from the estimate.
If your cycles are usually longer than 28 days, conception probably happened a little later than December 21; 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.