How the conception date is estimated
Conception happens at ovulation — the egg is only viable for about 12–24 hours — so estimating conception means estimating ovulation. The arithmetic is simple in each direction:
The 266 comes from the standard 280-day pregnancy minus the ~14 days between a period and ovulation. Working back from a birth date is the fuzziest of the three: babies arrive anywhere from 37 to 42 weeks, so the true conception day can sit more than a week either side of the estimate. Working back from a due date set by an early ultrasound is the tightest.
Why conception is a window, not a day
Two pieces of biology smear the date. First, ovulation moves— even in regular cycles it can shift a few days with stress, illness or travel, and the “14 days before the next period” rule is an average, not a law. Second, the gametes have very different lifespans:
| Cell | Survives | What it means for dating |
|---|---|---|
| Sperm | up to 5 days | Sex up to five days before ovulation can cause the pregnancy — the act and the conception can be nearly a week apart |
| Egg | 12–24 hours | Fertilization itself is pinned tightly to ovulation day — the uncertainty is in when ovulation was, not what happened after |
Put together: the conception day is ovulation day give or take very little, but the sex that led to it could have been any day in the six-day fertile window before it. The calculator shows both, because they answer different questions. About 6–12 days after conception the embryo implants in the uterine lining and hCG production starts — the reason a pregnancy test only becomes reliable around the missed period.
How ultrasound refines the estimate
A first-trimester ultrasound measures the embryo’s crown-rump length, which grows so uniformly in early pregnancy that it dates the pregnancy to within about five days — better than any calendar method. When a provider redates a pregnancy after an early scan, the conception estimate should move with it: take the ultrasound-based due date and subtract 266 days. What no scan can do is turn the window into a single day; it narrows the error bars, it doesn’t remove them.
An honest note on paternity questions
A common reason people reach for this calculator is to work out who the father is. The honest answer: this window rarely settles it. Between the multi-day fertile window, five-day sperm survival and the error in cycle-based dating, two candidate partners within about a week of each other simply cannot be separated by arithmetic. If the question matters, a DNA paternity test is the only reliable route — non-invasive prenatal versions work from maternal blood from roughly 7–9 weeks. A calculator should tell you what it can’t do, and this is the biggest thing this one can’t.
Related tools
If you’re looking forward rather than backward, the ovulation calculator maps the fertile window for upcoming cycles, and the conception calculator ranks the best days to try. Already pregnant? The due date calculator runs today’s arithmetic in the forward direction, and the pregnancy calculator turns the date into a week-by-week timeline. As with all of these: estimates for orientation, not medical advice.