When Did I Conceive if My Due Date Is May 18?
A due date of May 18 points back to a conception date of about August 25 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 May 18 due date
| Most likely conception date | August 25 |
|---|---|
| Likely conception window | August 22 – August 28 |
| Days when sex could have led to pregnancy | August 20 – August 26 |
| Estimated first day of last period | August 11 |
| Positive home test likely from | September 8 (around the missed period) |
| Second trimester begins (14 weeks) | November 17 |
| Third trimester begins (28 weeks) | February 23 |
| Full term begins (39 weeks) | May 11 |
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 May 18 gives August 25.
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 August 25; 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.