When Did I Conceive if My Due Date Is September 21?
Wondering when your September 21 baby was conceived? Using the standard 266-day rule, the answer is most likely December 29 of the previous year, give or take a few days.
Key dates for a September 21 due date
| Most likely conception date | December 29 |
|---|---|
| Likely conception window | December 26 – January 1 |
| Days when sex could have led to pregnancy | December 24 – December 30 |
| Estimated first day of last period | December 15 |
| Positive home test likely from | January 12 (around the missed period) |
| Second trimester begins (14 weeks) | March 23 |
| Third trimester begins (28 weeks) | June 29 |
| Full term begins (39 weeks) | September 14 |
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 21 gives December 29.
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?
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 29; 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.