Due on December 28? Your Most Likely Conception Date
A due date of December 28 points back to a conception date of about April 6. 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 28 due date
| Most likely conception date | April 6 |
|---|---|
| Likely conception window | April 3 – April 9 |
| Days when sex could have led to pregnancy | April 1 – April 7 |
| Estimated first day of last period | March 23 |
| Positive home test likely from | April 20 (around the missed period) |
| Second trimester begins (14 weeks) | June 29 |
| Third trimester begins (28 weeks) | October 5 |
| Full term begins (39 weeks) | December 21 |
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 28 gives April 6.
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 April 6; 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.