When Did I Conceive if My Due Date Is December 23?
If your baby is due on December 23, conception most likely happened around April 1. Here is how that date is worked out and the range of days it could realistically fall on.
Key dates for a December 23 due date
| Most likely conception date | April 1 |
|---|---|
| Likely conception window | March 29 – April 4 |
| Days when sex could have led to pregnancy | March 27 – April 2 |
| Estimated first day of last period | March 18 |
| Positive home test likely from | April 15 (around the missed period) |
| Second trimester begins (14 weeks) | June 24 |
| Third trimester begins (28 weeks) | September 30 |
| Full term begins (39 weeks) | December 16 |
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 23 gives April 1.
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?
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 April 1; 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.