Due on March 24? Your Most Likely Conception Date
Counting back from a March 24 due date puts the most likely day of conception at July 1 of the previous year. That's an estimate, so we've also included the window of days around it.
Key dates for a March 24 due date
| Most likely conception date | July 1 |
|---|---|
| Likely conception window | June 28 – July 4 |
| Days when sex could have led to pregnancy | June 26 – July 2 |
| Estimated first day of last period | June 17 |
| Positive home test likely from | July 15 (around the missed period) |
| Second trimester begins (14 weeks) | September 23 |
| Third trimester begins (28 weeks) | December 30 |
| Full term begins (39 weeks) | March 17 |
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 24 gives July 1.
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?
Due dates set from an early ultrasound are usually more accurate than those set from a period date alone, so if your provider changed your due date after a scan, use the newer one here.
If your cycles are usually longer than 28 days, conception probably happened a little later than July 1; 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.