Reverse Due Date Calculator (Find Your Conception Date)
Enter a due date or your baby's birth date and see the likely conception day, the intercourse window that could explain it, and your implied last menstrual period.
Quick samples
Enter a date to work backwards.
Choose a starting point, add your due or birth date (or try a quick sample), and your likely conception date appears here.
Conception is estimated as 266 days before your due date (or birth date), and the implied last period as 280 days before it. Only an early scan can date a pregnancy precisely.
The formula
Conception ≈ due date − 266 days · implied LMP ≈ due date − 280 days · intercourse window ≈ conception − 5 to conception
- 266 days — Thirty-eight weeks — the average span from conception to birth.
- 280 days — Forty weeks from the last menstrual period, the clinical due-date count.
- LMP — First day of the last period, the reference point pregnancy is dated from.
- − 5 days — How long sperm can survive, so intercourse in the days before ovulation can lead to conception.
The two anchors, at a glance
| Quantity | Days before due date | What it marks |
|---|---|---|
| Last menstrual period (LMP) | 280 | Start of the clinical count, about two weeks pre-ovulation |
| Estimated conception | 266 | Fertilisation of the egg, when ovulation is typical |
| Fertile intercourse window | 261 to 266 | Sperm survival days leading up to conception |
| Full-term birth range | due − 21 to due + 13 | 37 to 41+6 weeks, when most babies arrive |
Day counts assume a textbook 28-day cycle; a longer or shorter cycle shifts ovulation and therefore conception.
A reverse due date calculator works the usual pregnancy maths backwards. Give it a due date, or the day your baby was actually born, and it estimates the date you most likely conceived, the window of intercourse that could have led to that conception, and the implied first day of your last period.
People reach for this in two situations. Some are pregnant with a known due date and want to understand when conception probably happened; others are looking back from a child's birthday and want a sense of the pregnancy timeline. Either way the same arithmetic applies, just pointed in the opposite direction.
The forward calculation is well established: a due date is 280 days after the last period, and conception typically lands about 266 days before it. Reversing those two constants gives a best estimate, though the result is a range rather than a single certain day.
How the reverse calculation works
A standard pregnancy lasts about forty weeks from the first day of the last menstrual period, or about thirty-eight weeks from conception. That gap exists because dating counts from the last period, roughly two weeks before ovulation and fertilisation usually occur.
To work backwards, the calculator subtracts 266 days from your due date to place the likely conception day, and 280 days to place the implied last period. These are the mirror images of the forward due-date rule.
Because sperm can live for up to five days inside the reproductive tract, the intercourse that caused a given conception could have happened several days before the estimated conception day. That is why the tool shows a window, not a single date.
Working from a birth date instead
If you know the day your baby was born but not your due date, the calculator treats the birth as roughly forty weeks and works back from there. It is the same subtraction, just anchored to the actual birth.
This version carries more uncertainty, because few babies arrive on their exact due date. Most are born somewhere between thirty-seven and forty-one weeks, so a conception date derived from a birth date can sit a week or two either side of the true one.
The result is still useful for understanding a rough timeline, and for comparing a family birthday to a school year or a due-date estimate. Where a precise date matters, a documented pregnancy due date beats a backwards calculation from birth.
A worked example
Suppose your due date is 8 March. Subtracting 266 days lands on 24 May, the estimated conception day, and subtracting 280 days gives an implied last period beginning 25 May of the previous cycle reference — about two weeks before conception.
The intercourse window then runs from roughly 19 May to 24 May, since sperm surviving up to five days could have fertilised the egg around the estimated conception date. Any of those days could plausibly be the one that led to pregnancy.
Read the other way, a baby born on 8 March without a known due date points to a similar May conception window, but with the added caveat that the birth itself may have been a little early or late relative to forty weeks.
How accurate a reversed date is
The maths is exact, but the inputs are not. A calendar due date is itself an estimate with a spread of days around it, so reversing it inherits that uncertainty and adds a little more.
Ovulation is the biggest source of error. The standard rule assumes it happens fourteen days before the next period, yet normal cycles vary, and a late or early ovulation shifts the true conception day even when the due date is fixed.
That is why the honest output is a window. Treat the estimated conception day as the centre of a plausible range rather than a pinpoint, and rely on an early ultrasound if you need dating you can act on medically.
When to trust a scan over dates
For anything that carries real consequences — a paternity question, a medical decision, or a disputed start date — a first-trimester dating scan is the reliable reference. Early scans measure the embryo directly and can pin gestational age to within a few days.
A calculator cannot match that precision because it only knows dates, not measurements. It is a sound way to understand your likely timeline, not a substitute for clinical dating.
If you have both a scan-due date and a period-based one, use the scan date in the calculator. The clinician already weighed the two and generally trusts the scan, so building your reverse estimate from it keeps everything consistent.
Using a reversed date to plan backwards
A reversed conception window is not only about looking back. Someone planning ahead can set a target due date and read the conception window and implied last period that would produce it, which turns a vague 'when should I start trying' question into a concrete stretch of days.
That is exactly how a family birthday lands against a school or nursery cut-off, or why a due date sits a month earlier or later than a friend's due in the same season. Seeing the dates line up in both directions makes the timeline legible.
It can also be a gentle reality check about timing. Because the window is only a handful of days, planning purely around a hoped-for birth month is imprecise, and a normal margin of a week or two either way is expected rather than a sign of error.
Key terms
- Reverse due date
- Working backwards from a due date or birth date to estimate conception and the last period.
- Conception date
- The day the egg was fertilised — usually about 266 days before a due date.
- Implied LMP
- The last-period start date a due date points back to, used as the dating reference.
- Fertile window
- The run of days when intercourse could lead to conception, ending on the ovulation day.
Pro tips
- If you have an early-scan due date, use that — it reverses to a far tighter conception window than a period-based date.
- Read the intercourse window, not just the single conception day, since sperm survive several days.
- Working from a birth date is a rougher tool; add a week either side to reflect normal birth timing.
Common mistakes
- Treating the estimated conception day as certain rather than the centre of a range.
- Forgetting that the due date you are reversing is itself an estimate.
- Assuming ovulation on day fourteen when a long or short cycle moves it.
Reverse due date calculator — your questions
Can you really calculate a conception date from a due date?
You can estimate it, not prove it. Because a due date is set at 280 days after the last period and conception normally falls about 266 days before that date, subtracting gives a likely conception day. The catch is that the due date is an estimate too, so the conception result carries the same spread and deserves to be read as a window rather than an exact day.
What is the difference between working from a due date and a birth date?
A due date is already a clean forty-week target, so reversing it is tidy. A birth date is a real event, but most babies are not born at exactly forty weeks, so working back from birth can misplace conception by a week or two depending on how early or late the baby arrived. Use a due date where you have one, and treat a birth-date result as approximate.
Why does the calculator show a window of dates, not one day?
Two reasons. Sperm can survive up to about five days in the reproductive tract, so intercourse well before ovulation can still result in conception. And the underlying ovulation date is itself uncertain. Combining those, the honest answer is a range of plausible days rather than a single date, which is exactly what the intercourse window represents.
How accurate is this for dating a pregnancy?
Accurate enough to understand a likely timeline, but not enough for anything you must rely on. Calendar and reversed maths can be off by days to a week or more. A first-trimester ultrasound measures the embryo directly and can date a pregnancy within a few days, so it remains the reference whenever precision genuinely matters.
Does this work if my cycles are irregular?
It works less well, because irregular cycles mean ovulation was not where the standard rule assumes. The implied last period may be far from your real one, and the conception estimate inherits that error. If your cycles are irregular or you cannot recall a date, a dating scan is the dependable route, and reversing the scan's due date is better than reversing a period calculation.
Is a reverse due date calculator useful for a due date I do not have yet?
Yes, in one direction. If you know a birth date and want the pregnancy timeline around it, reversing gives the implied last period and conception window, which is handy for records or reflection. It cannot predict a future due date for a current pregnancy, though; for that, use the forward pregnancy due date calculator with your last period or a scan date.