When Did I Conceive if My Due Date Is December 20?
A due date of December 20 points back to a conception date of about March 29. 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 20 due date
| Most likely conception date | March 29 |
|---|---|
| Likely conception window | March 26 – April 1 |
| Days when sex could have led to pregnancy | March 24 – March 30 |
| Estimated first day of last period | March 15 |
| Positive home test likely from | April 12 (around the missed period) |
| Second trimester begins (14 weeks) | June 21 |
| Third trimester begins (28 weeks) | September 27 |
| Full term begins (39 weeks) | December 13 |
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 20 gives March 29.
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?
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 29; 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.