What Is the Reverse Due Date Calculator?
A reverse due date calculator works backward from your estimated due date to pinpoint when conception likely happened. By subtracting 266 days from your due date, it estimates your conception date, and by subtracting 280 days, it estimates the first day of your last menstrual period.
Most pregnancy calculators run forward: you enter your last period and they project a due date. This one runs in reverse. Enter a due date — one your doctor gave you after an ultrasound, or a target birth date you are planning around — and the tool rewinds the clock. You get three results back: the estimated day of conception, a small window around it, and the estimated first day of your last menstrual period. Those three dates together reconstruct the early timeline of a pregnancy.
People use it for all kinds of reasons. Some are simply curious about their pregnancy’s beginning — roughly when it started. Others are planners: if you would love a spring baby, the calculator shows which weeks of the year to aim for. And if you conceived around a known date, the reversed LMP can help you sense-check a due date you were given elsewhere. Whatever your reason, think of it as a planning and curiosity tool — a well-grounded estimate, not a medical record.
How to Use the Reverse Due Date Calculator
The tool needs just one input from you. If you are planning ahead, use the target due date you have in mind; if you are already pregnant, use the due date your provider gave you.
- Enter your due date using the month, day, and year selectors. This is the only input the calculation needs.
- Run the calculation to instantly generate your reversed timeline — no account, no waiting.
- Read your three results: the estimated conception date, the conception window around it, and the estimated first day of your last menstrual period.
- Use the window for planning. If you are timing conception for a target birth month, the window shows the days to aim for — and remember that sperm can survive several days, so the fertile window starts a few days earlier than the estimate suggests.
Once you have your dates, you can explore what those early weeks look like in our week-by-week pregnancy guide, which walks through each stage from the very first week.
How the Calculation Works
The math is simply the standard pregnancy dating formula run in reverse. Doctors date pregnancies from the first day of the last menstrual period using Naegele’s rule: due date = LMP + 280 days (40 weeks). Flip that around and you get the two core formulas:
The 14-day difference between the two formulas reflects the average follicular phase: in a typical 28-day cycle, ovulation — and therefore fertilization — happens about two weeks after the last period began. That is also why a pregnancy is already considered “two weeks along” at the very moment of conception. The small window around the conception estimate (three days before to one day after) acknowledges that ovulation does not land on the same calendar day in every cycle.
One more piece of biology worth knowing: sperm can survive up to five days in the reproductive tract while an egg is viable for roughly 24 hours. That means the intercourse that led to conception may have happened several days before the estimated conception date itself. The estimate marks fertilization, not necessarily the date of intercourse — a distinction that matters if you are comparing the results with your memory of events.
Worked Example
Let us walk through a real calculation. Suppose your due date is Thursday, October 8, 2026. The calculator rewinds from that date:
- Estimated LMP: October 8, 2026 − 280 days = Thursday, January 1, 2026
- Estimated conception: October 8, 2026 − 266 days = Thursday, January 15, 2026
- Conception window: January 12–16, 2026 (three days before to one day after the estimate)
So a baby due on October 8, 2026 was most likely conceived around January 15, 2026, with the last period starting around New Year’s Day. Now flip it into planning mode: say you would love a June baby. Enter a target due date of Tuesday, June 15, 2027, and the math gives an estimated conception date of Tuesday, September 22, 2026 — meaning late September is the window to aim for.
Notice how the window stays small but the meaning changes with your goal. For curiosity, the single estimated date is the headline. For planning, the window is what matters, because you cannot schedule fertilization to the day — you can only aim your fertile days at it. If you want to pin down ovulation more precisely, pair this with an ovulation-based due date calculator.
How Accurate Is It?
The reverse calculation is only as accurate as the due date you enter — and due dates themselves are estimates. Research suggests only about 4–5% of babies are born on their exact due date, with most arriving within a couple of weeks on either side. So the conception estimate typically lands within about five days of the true date, and the reversed LMP within a similar band.
The biggest source of error is ovulation timing. The 266-day figure assumes ovulation on day 14 of a 28-day cycle, but studies estimate that in real cycles ovulation commonly falls anywhere from day 10 to day 20, and sometimes outside that range. If your cycles run long or short, the estimate shifts accordingly. Early ultrasound dating (before about 14 weeks) is generally the most accurate way to date a pregnancy, which is why entering a scan-based due date gives you the most trustworthy reversed results.
None of this makes the tool unreliable — it makes it honest. For curiosity, planning a birth month, or checking a timeline for consistency, ±5 days is plenty precise. For anything clinical, like dating for medical decisions, your provider’s ultrasound-based dating is the standard to follow.
Irregular cycles deserve a special note, because they are the most common reason reversed dates feel “off.” If your cycles vary in length, ovulation does not reliably fall on day 14, and the 266-day assumption can miss by a week or more. In that case the reversed conception date is best read as the middle of a wider window rather than a single day — or better yet, replaced by dating from an early ultrasound, which measures the embryo directly instead of assuming a textbook cycle.
When to Talk to Your Doctor
A reverse calculator is a planning aid, not a substitute for medical guidance. Bring your timeline to your provider — especially early in pregnancy — if the reversed dates do not line up with an ultrasound estimate, if you are unsure about your cycle timing, or if anything about the pregnancy feels uncertain. Discrepancies are usually just biology being variable, but they are worth a professional look.
If you are using the calculator to plan a pregnancy, the standard guidance is to see your doctor if you have been trying for 12 months without success when under 35, or for 6 months when 35 or older — or sooner if you have irregular cycles, known fertility conditions, or concerns about your health. A preconception visit is also a good moment to discuss folic acid, vaccinations, and any medications, so your planning starts on the strongest footing.
This calculator provides estimates for educational purposes and is not medical advice. Always confirm with your healthcare provider.