Due on March 15? Your Most Likely Conception Date
A due date of March 15 points back to a conception date of about June 22 of the previous year. 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 March 15 due date
| Most likely conception date | June 22 |
|---|---|
| Likely conception window | June 19 – June 25 |
| Days when sex could have led to pregnancy | June 17 – June 23 |
| Estimated first day of last period | June 8 |
| Positive home test likely from | July 6 (around the missed period) |
| Second trimester begins (14 weeks) | September 14 |
| Third trimester begins (28 weeks) | December 21 |
| Full term begins (39 weeks) | March 8 |
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 March 15 gives June 22.
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 June 22; 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.