Due on December 31? Your Most Likely Conception Date
If your baby is due on December 31, conception most likely happened around April 9. Here is how that date is worked out and the range of days it could realistically fall on.
Key dates for a December 31 due date
| Most likely conception date | April 9 |
|---|---|
| Likely conception window | April 6 – April 12 |
| Days when sex could have led to pregnancy | April 4 – April 10 |
| Estimated first day of last period | March 26 |
| Positive home test likely from | April 23 (around the missed period) |
| Second trimester begins (14 weeks) | July 2 |
| Third trimester begins (28 weeks) | October 8 |
| Full term begins (39 weeks) | December 24 |
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 31 gives April 9.
Because sperm can live for up to five days, the act of sex and the moment of conception aren't always the same day. Fertilisation happens close to ovulation, but the sex that led to it may have been days earlier.
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 9; 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.