Reverse Due Date Calculator
Work backward from a due date, a baby’s birth date, or a birth month you’re planning for — and see the likely conception date, fertile window, last menstrual period, implantation window, and full pregnancy timeline.
Curious when you or your child were conceived? Enter a birth date. If the baby was early or late, the estimate shifts accordingly.
We’ll show the conception window you’d aim for to have a baby due in that month.
Enter a date above to work backward to conception, your fertile window, and your last period.
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Your pregnancy timeline, in reverse
Trimester dates
These are statistical estimates, not exact dates. Conception depends on when you actually ovulated, and sperm can survive up to five days — so the real conception day can fall anywhere in the window shown. Only about 1 in 20 babies arrive on the due date. For anything clinical or legal, please rely on your provider, not this tool.
How the reverse due date calculator works
A normal due date calculator runs forward: you give it your last period and it adds 40 weeks. A reverse due date calculator does the opposite — you already know the due date (or a birth date), and it works backward to reveal when conception likely happened, when your last period was, and the fertile window around it. It’s the same clinical math, simply inverted.
The key numbers are 280 and 266. Pregnancy is dated as 280 days (40 weeks) from the first day of the last menstrual period, but conception happens about two weeks later — roughly 266 days (38 weeks) before the due date. So this tool subtracts 266 days from your due date to estimate conception, and 280 days to estimate your last period, adjusting for your cycle length. If you’d rather run it the normal way, our Pregnancy Due Date Calculator goes forward instead.
Quick answer
To find your conception date from your due date, subtract 266 days (38 weeks) from the due date. To estimate your last menstrual period, subtract 280 days (40 weeks). Because ovulation and sperm survival vary, treat conception as a window of about a week, not a single day.
Three ways to use it
| Mode | You enter | You get | Great for |
|---|---|---|---|
| From a due date | Your EDD | Conception, LMP, fertile window | Knowing when you conceived |
| From a birth date | Any birth date | Conception date & day of week | Curiosity about you or your child |
| Plan a birth month | Target month & year | The conception window to aim for | Planning a spring or summer baby |
Why conception is a window, not a day
Even with a perfect due date, the exact moment of conception can’t be pinned down. Sperm can survive in the body for up to five days, and the egg is viable for about a day, so the intercourse that led to pregnancy could have happened several days before conception actually occurred. That’s why this calculator shows a fertile and intercourse window around the estimated conception date, plus an implantation window six to twelve days later. If you tracked ovulation, you can cross-check the timing with our Basal Body Temperature Charting Tool.
How cycle length changes the result
The standard math assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, you ovulate later, so your last period would have been earlier relative to conception — and the calculator shifts your LMP estimate accordingly while keeping conception anchored near 266 days before the due date. If your due date itself came from an early ultrasound rather than your last period, it’s usually the more reliable starting point; you can confirm gestational age with our Ultrasound Gestational Age Calculator or the Crown-Rump Length Calculator.
A worked example
Suppose your due date is 6 December and you have a 28-day cycle. Subtract 266 days and conception lands around 15 March; subtract 280 and your last period was near 1 March. The fertile window would have been roughly 10–16 March, and implantation around 21–27 March. Shift to a 32-day cycle and your last period moves a few days earlier while conception stays put — because you’d have ovulated later in that longer cycle.
Related maternal health calculators
This tool is part of the Maternal Health Calculators hub. These companion tools cover cycle tracking, ultrasound dating, and maternity planning — each answers a different question, so pick whichever fits where you are.
Frequently asked questions
Subtract 266 days (38 weeks) from your due date. That lands on your estimated conception date. This calculator does it for you and also shows a window of about a week, since the exact day varies.
Your last menstrual period (LMP) is about two weeks before conception. Pregnancy is dated from the LMP as 280 days, but conception itself is roughly 266 days before the due date. So this tool shows both, 14 days apart on a standard cycle.
It’s a good estimate, but not exact. It assumes a standard cycle and average ovulation, and the real conception day can fall a few days either way. If your due date came from an early ultrasound, the estimate is more reliable.
Yes — use the “From a birth date” mode and enter your birthday. The calculator subtracts 266 days and even shows the day of the week. If you were born early or late, the estimate shifts to match.
Use the “Plan a birth month” mode. Pick your target month and year, and the tool shows the conception window you’d aim for. Remember conception isn’t guaranteed in any single cycle, so treat it as a target, not a promise.
Yes. Longer cycles mean later ovulation, which shifts your estimated last period earlier relative to conception. Set your average cycle length with the slider so the LMP estimate matches your body.
Because sperm can survive up to five days and the egg lasts about a day, the intercourse that led to pregnancy could have been several days before conception. A window reflects that biological reality better than a single date.
Implantation usually occurs 6 to 12 days after conception, when the fertilised egg attaches to the uterine lining. The calculator estimates this window for you, though it isn’t the same as conception itself.
The general timeline still applies, but IVF dating is more precise because the embryo age is known. If you conceived through IVF, our IVF Due Date Calculator gives a more exact conception and due date.
No. Reverse dating is inherently approximate. For anything clinical or legal, rely on your healthcare provider and formal dating such as an early ultrasound rather than an online estimate.
Methodology
This calculator applies Naegele’s rule in reverse. From an estimated due date (or birth date), it subtracts 266 days (38 weeks) to estimate the conception date and 280 days (40 weeks) to estimate the first day of the last menstrual period, adjusting the last-period estimate by the difference between your average cycle length and 28 days. Conception is presented as a window of roughly a week to reflect that sperm can survive up to five days and the egg about one day. The fertile and intercourse window spans about five days before to one day after estimated ovulation, and the implantation window is estimated at 6 to 12 days after conception. The birth-month planning mode targets the middle of the chosen month and reports the corresponding conception window. All results are population-level estimates for education and planning, not exact dates, and are not a substitute for care from a qualified provider or for formal clinical or legal dating.
Reviewer
References & trusted sources
- ACOG — Methods for Estimating the Due Date (Committee Opinion 700)
- NHS — Working out your due date
- MedlinePlus — Pregnancy and prenatal care
- StatPearls / NCBI — Estimated Date of Delivery (Naegele’s rule)
Last reviewed & updated: July 2026.