Due on March 21? Your Most Likely Conception Date
Wondering when your March 21 baby was conceived? Using the standard 266-day rule, the answer is most likely June 28 of the previous year, give or take a few days.
Key dates for a March 21 due date
| Most likely conception date | June 28 |
|---|---|
| Likely conception window | June 25 – July 1 |
| Days when sex could have led to pregnancy | June 23 – June 29 |
| Estimated first day of last period | June 14 |
| Positive home test likely from | July 12 (around the missed period) |
| Second trimester begins (14 weeks) | September 20 |
| Third trimester begins (28 weeks) | December 27 |
| Full term begins (39 weeks) | March 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 March 21 gives June 28.
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?
Remember that a due date is only the midpoint of a range. Ovulation timing varies from cycle to cycle, so the true conception day can sit a few days away from the estimate.
If your cycles are usually longer than 28 days, conception probably happened a little later than June 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.