You know the due date — this works out the rest. Enter it to see when you likely conceived, when the sex that led to pregnancy could have happened, and your implied last period.
The date from your provider, your first scan, or a due date calculator. It can also be a past due date if your baby has already arrived.
Every due date is built from one formula: 280 days (40 weeks) after the first day of the last menstrual period. Running that formula in reverse unlocks the whole timeline. Subtract 280 days from the due date and you land on the implied last period. Move forward about two weeks from there — the typical gap before ovulation — and you reach the estimated conception date, 266 days before the due date. This calculator does both subtractions and adds the biological context around them.
Why the window around conception? Two reasons. First, ovulation doesn't run on rails — even in textbook cycles it can land a few days early or late, and the due date itself may have been rounded by dating conventions. Second, fertilization can only happen in the roughly 24 hours after ovulation, but the intercourse that causes it can precede it by up to five days, because sperm survive that long. So "when did I conceive?" and "when was the sex that caused it?" genuinely have different answers — the first is a date with a margin, the second is a range ending on that date.
Like every tool on this site, the calculation runs entirely in your browser. Nothing you enter is stored, sent or shared.
A reverse calculation is great for satisfying curiosity ("was it the holiday trip?"), for filling in a pregnancy journal, or for sanity-checking dates against what you remember of your cycle. Where it reaches its limit — and it's important to be honest about this — is in separating possibilities that are close together. If two candidate occasions fall within about a week of each other, their conception windows overlap, and no amount of date arithmetic can tell them apart. In particular, a reverse due date calculator cannot establish paternity. Only DNA testing can do that, and providers can arrange it both during and after pregnancy.
There is one case where backward math becomes genuinely precise: IVF. With a known egg retrieval and embryo transfer date, the fertilization date isn't estimated — it's recorded. If you conceived through IVF, your clinic's dates are exact and this page's estimates are unnecessary.
One more caveat worth repeating: the output inherits the quality of the input. A due date set by an early ultrasound produces a tighter, more trustworthy backward timeline than one calculated from a shaky period date. If you have both, use the scan-based date.
Estimated conception date — the single most likely day, 266 days before your due date, shown with its ±5-day window. Sex that led to conception — the span from five days before the estimated conception date through the date itself, reflecting how long sperm can wait for an egg. Implied last period — 280 days before the due date; useful for checking the estimate against your own records. If the due date you entered is still ahead of you, the tool also shows your current gestational age, counted on the same clock your provider uses.
If the conception window doesn't line up with what you remember, don't panic — check whether your due date was ever adjusted after a scan, and whether your cycles run long or short. When in doubt, your provider can walk through the dating with you.
It runs the standard pregnancy math backward. A due date is set at 280 days (40 weeks) after the first day of the last menstrual period, and conception typically happens about 14 days into that count. So the calculator subtracts 280 days from your due date to find the implied last period, and subtracts 266 days (38 weeks) to find the estimated conception date. Because ovulation timing varies from cycle to cycle, the true conception date most likely falls within a window of several days around that estimate.
Not exactly — and no calculator can. Conception depends on when you ovulated, which varies even in regular cycles, and fertilization can happen hours to a day after ovulation. What you can establish is a strong likely window: the estimated conception date plus or minus about five days covers the realistic range for most pregnancies. An IVF pregnancy is the exception, where the fertilization and transfer dates are known precisely.
It could have been on the estimated conception date or up to about five days before it. Sperm can survive in the reproductive tract for around five days, so intercourse well before ovulation can still cause pregnancy. That means the act of intercourse that led to conception may have happened days before the egg was actually fertilized — one more reason conception dates are windows, not single days.
No. If two possible partners were within roughly a week of each other, the conception windows will overlap and no date-based calculation can separate them — the math simply is not precise enough, and it should never be used to settle legal or personal paternity questions. Only DNA testing can determine paternity. If this matters to you, talk with your healthcare provider about testing options during or after pregnancy.
The result is only as good as the due date you start from. A due date confirmed or adjusted by a first-trimester ultrasound is the most reliable input, because early scans date pregnancies most accurately. If your due date came only from period math and your cycles are irregular, both the due date and the conception estimate inherit that uncertainty. Whichever way it was set, remember that only about 1 in 20 babies actually arrives on the due date itself.