Estimate when you likely conceived, and see how the two common calculation methods compare.
Estimate only, not medical advice. Ovulation and conception timing vary by cycle.
There are two ordinary ways to back into a conception date, and this calculator runs whichever one matches what you already know.
| Starting point | Formula | Why |
|---|---|---|
| Due date | Due date minus 266 days | A full term pregnancy runs about 266 days from conception, roughly 38 weeks |
| Last menstrual period | LMP plus about 14 days | Ovulation and conception typically occur around day 14 of a 28 day cycle |
In a textbook 28 day cycle both formulas point to the same day. When they diverge slightly, it is usually because the actual cycle behind the pregnancy ran longer or shorter than the 28 day assumption baked into the LMP method.
An egg is viable for roughly a day after ovulation, but sperm can survive inside the reproductive tract for several days beforehand, which means intercourse on any of several days around ovulation can result in conception. Cycle length also drifts naturally from one month to the next. Combined, these two sources of variation are why this calculator surrounds its best estimate with a window of roughly plus or minus five days rather than reporting a single date.
Conception is the moment an egg is fertilized. Implantation, when the resulting embryo attaches to the uterine lining, is a separate, later event, typically about 6 to 10 days after conception. A home pregnancy test cannot detect anything until after implantation, because the hormone those tests measure is only produced once the embryo has implanted. If you are trying to work out an early testing date rather than a conception date, the implantation calculator on this site walks through that separately.
Counting back from a due date and counting forward from your last period both rest on the same 280 day span, so in a textbook 28 day cycle they agree. Any difference usually means your actual cycle runs longer or shorter than 28 days.
Ovulation timing shifts naturally from cycle to cycle, and sperm can remain viable for several days before an egg is released, so any single conception day is an estimate within a small range.
Roughly, yes. Because pregnancy is dated from your last period rather than conception, ovulation and conception typically fall around what is labeled week 2 of gestational age.
This tool works from calendar dates alone. Temperature charting or ovulation predictor kits observe your body directly and can narrow the estimate further than date math alone.