When Did I Conceive if My Due Date Is September 30?
If your baby is due on September 30, conception most likely happened around January 7. Here is how that date is worked out and the range of days it could realistically fall on.
Key dates for a September 30 due date
| Most likely conception date | January 7 |
|---|---|
| Likely conception window | January 4 – January 10 |
| Days when sex could have led to pregnancy | January 2 – January 8 |
| Estimated first day of last period | December 24 |
| Positive home test likely from | January 21 (around the missed period) |
| Second trimester begins (14 weeks) | April 1 |
| Third trimester begins (28 weeks) | July 8 |
| Full term begins (39 weeks) | September 23 |
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 September 30 gives January 7.
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?
Only a small share of babies, often quoted at around 4 to 5 percent, arrive on their exact due date, so dates worked backwards from a due date carry a margin of a few days either way.
If your cycles are usually longer than 28 days, conception probably happened a little later than January 7; 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.