When Did I Conceive if My Due Date Is October 21?
A due date of October 21 points back to a conception date of about January 28. 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 October 21 due date
| Most likely conception date | January 28 |
|---|---|
| Likely conception window | January 25 – January 31 |
| Days when sex could have led to pregnancy | January 23 – January 29 |
| Estimated first day of last period | January 14 |
| Positive home test likely from | February 11 (around the missed period) |
| Second trimester begins (14 weeks) | April 22 |
| Third trimester begins (28 weeks) | July 29 |
| Full term begins (39 weeks) | October 14 |
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 October 21 gives January 28.
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 January 28; 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.