Due on December 16? Your Most Likely Conception Date
A due date of December 16 points back to a conception date of about March 25. 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 16 due date
| Most likely conception date | March 25 |
|---|---|
| Likely conception window | March 22 – March 28 |
| Days when sex could have led to pregnancy | March 20 – March 26 |
| Estimated first day of last period | March 11 |
| Positive home test likely from | April 8 (around the missed period) |
| Second trimester begins (14 weeks) | June 17 |
| Third trimester begins (28 weeks) | September 23 |
| Full term begins (39 weeks) | December 9 |
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 16 gives March 25.
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?
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 25; 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.