Due Date December 12: When Did You Conceive?
A due date of December 12 points back to a conception date of about March 21. Below you'll find the full set of dates that go with it, from your estimated last period to the start of each trimester.
Key dates for a December 12 due date
| Most likely conception date | March 21 |
|---|---|
| Likely conception window | March 18 – March 24 |
| Days when sex could have led to pregnancy | March 16 – March 22 |
| Estimated first day of last period | March 7 |
| Positive home test likely from | April 4 (around the missed period) |
| Second trimester begins (14 weeks) | June 13 |
| Third trimester begins (28 weeks) | September 19 |
| Full term begins (39 weeks) | December 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 December 12 gives March 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?
A due date is a planning estimate rather than a promise. Most babies arrive within two weeks either side of it, which is why any conception date calculated from it is a best guess.
If your cycles are usually longer than 28 days, conception probably happened a little later than March 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.