Due Date December 15: When Did You Conceive?
If your baby is due on December 15, conception most likely happened around March 24. Here is how that date is worked out and the range of days it could realistically fall on.
Key dates for a December 15 due date
| Most likely conception date | March 24 |
|---|---|
| Likely conception window | March 21 – March 27 |
| Days when sex could have led to pregnancy | March 19 – March 25 |
| Estimated first day of last period | March 10 |
| Positive home test likely from | April 7 (around the missed period) |
| Second trimester begins (14 weeks) | June 16 |
| Third trimester begins (28 weeks) | September 22 |
| Full term begins (39 weeks) | December 8 |
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 15 gives March 24.
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?
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 24; 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.