Updated June 17, 2026
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In a standard 28-day cycle, conception occurs around day 14 after the first day of your last menstrual period (LMP), which is also roughly 266 days (38 weeks) before your due date. To estimate when you conceived, subtract 266 days from your due date, or add 14 days to your LMP. These are estimates; the actual conception window spans the few days around ovulation.
Method 1: Work back from your due date
Your due date is set 280 days (40 weeks) from LMP. Since ovulation typically happens 14 days after LMP, conception is approximately 280 minus 14, or 266 days before your due date. Subtract 266 days from your due date to get an estimated conception date. The Conception Calculator on this site does this calculation for you.
Method 2: Add from your last period
Add 14 days to the first day of your LMP to estimate ovulation, which is also roughly the conception window. If your cycle is longer or shorter than 28 days, adjust: for a 32-day cycle, ovulation is more likely around day 18, so add 18 days instead. The result is a 2 to 3-day window, not an exact day, because sperm can survive for several days and fertilization can occur up to about 24 hours after ovulation.
Why the conception date is a range, not a single day
Sperm can live in the female reproductive tract for 3 to 5 days. An egg survives roughly 12 to 24 hours after ovulation. So intercourse anywhere from 5 days before to 1 day after ovulation can result in a pregnancy. This is why a conception "date" is really a window of several days, and pinning it to a single day is often not possible without fertility tracking or a very early ultrasound.
How accurate is the estimated conception date?
The estimate is reasonably close if your cycles are regular and your LMP is known. It can be off by several days for people with irregular cycles. An early ultrasound gives a fetal age that can be back-calculated to a more accurate conception window, though even that has a margin of a few days. See how due dates are calculated for the underlying method.
Conception date estimates are general calculations and not medical determinations. If you need a precise date for any medical or legal reason, discuss it with your doctor or midwife.
Find your due date
Get your estimated due date and current week in seconds.