Key takeaways
- Work backwards from a known due date to find your estimated LMP, conception window, and current gestational age.
- Useful when your provider has set an EDD by ultrasound but you want to reconcile it with your tracked cycle data.
- Also useful for planning: enter your target due date and see when conception would need to occur.
- LMP = EDD − 280 days. Estimated conception = EDD − 266 days. Both are computed and shown alongside trimester boundaries.
Why reverse dating is useful
Most pregnancy dating goes one direction: you enter your last menstrual period or your ultrasound, and the calculator returns an estimated due date. Reverse dating runs the formula the other way — you start with a known EDD and back-calculate the implied LMP, conception window, and current gestational age. There are two main reasons users want this: reconciling an ultrasound-set EDD with their cycle data, and planning a pregnancy around a target month.
The math is straightforward. Full-term gestation averages 280 days from LMP, or 266 days from conception. So:
- Estimated LMP = EDD − 280 days
- Estimated conception = EDD − 266 days (i.e., LMP + 14 days)
- Conception window = the 5–6 day fertile span around the estimated conception date
- Gestational age today = (today's date − estimated LMP) in weeks and days
Reconciling an ultrasound EDD with your cycle data
If your provider set your EDD by first-trimester ultrasound (typically a CRL measurement between 7 and 12 weeks), the EDD is already corrected for any cycle-length variation — and ACOG Committee Opinion 700 says ultrasound dating should override LMP when they disagree by more than 5–7 days. Running the EDD backwards gives you the implied LMP, which may not match the LMP you actually tracked. That gap is information: it tells you how much your real ovulation timing differed from the textbook day-14 assumption.
For long-cycle users, post-pill users, and PCOS users, the gap is often substantial — a week or more is common. The ultrasound-derived EDD is the right number to use for the rest of the pregnancy regardless. See our companion piece on first-trimester CRL ultrasound for how the formula works.
Planning a pregnancy around a target due date
If you have a target month — usually for school-year alignment, work-schedule reasons, or family-event reasons — reverse dating tells you when conception needs to occur. Subtract 266 days from your target EDD to find the estimated conception date. From there, use the Ovulation Calculator to identify the matching fertile window in your cycle.
Realistic expectation-setting matters here. Even healthy couples in their 20s and early 30s have roughly a 20% chance of conception per cycle when they time intercourse well. The Wilcox et al. (1995, NEJM) study found that 95% of conceptions in their cohort occurred within a six-day fertile window centered on ovulation. Plan for several months of trying around your target conception window, and read the fertile window explained for the underlying reproductive biology. If you are over 35 or have known fertility risk factors, talk to a reproductive endocrinologist before relying on calendar planning alone.
The conception window — why it is a window, not a date
Conception itself spans a window. Sperm survive in the female reproductive tract for up to 5 days under good cervical-mucus conditions. The egg, once released, is viable for 12–24 hours. So fertilization can occur up to 5 days after intercourse if intercourse preceded ovulation. Practical implication: the "conception date" implied by your EDD is not a single day. It is the center of a roughly 6-day band where fertilization is most likely to have happened. The calculator displays both the central estimate and the conception window for honesty.
When the reverse calculation is most reliable
Reverse dating from an EDD is most reliable when the original EDD was set by IVF dating (precise to the day), then by first-trimester ultrasound at 7–12 weeks (±5 days), then by LMP-based Naegele's rule (±7 days for regular cycles, less reliable for irregular). If your EDD was set by a late ultrasound (after 22 weeks), the underlying dating uncertainty is ±10–14 days — and that uncertainty propagates into your reverse-calculated LMP and conception date. Read more on the IVF dating method for the gold-standard case.
Frequently asked questions
How do I calculate LMP from a due date? +
Subtract 280 days (40 weeks) from the EDD to get the estimated last menstrual period. EDD = LMP + 280, so LMP = EDD − 280. This is the inverse of Naegele's rule and assumes a 28-day cycle. If your real cycle is longer or shorter, the back-calculated LMP will be a few days off your actual LMP — which is normal and expected, since the EDD-setting ultrasound has already corrected for cycle length.
How do I calculate the conception date from a due date? +
Subtract 266 days (38 weeks) from the EDD. Conception happens approximately 14 days after LMP in a typical 28-day cycle, so it is 280 − 14 = 266 days before the EDD. The actual conception event is usually within ±5 days of this estimate; sperm can survive up to 5 days in the reproductive tract, and the egg lasts 12–24 hours, so the precise conception date for any given pregnancy is rarely knowable to the day.
My provider set an EDD by ultrasound. Why does my back-calculated LMP differ from my real LMP? +
This is normal and informative. First-trimester ultrasound dating overrides LMP-based dating when they disagree by more than 5–7 days (ACOG guidance). The reason is usually that ovulation did not happen on cycle day 14 — most commonly because your cycle is longer or shorter than 28 days, or because of a single delayed-ovulation cycle. The back-calculated LMP from the ultrasound-derived EDD is what your LMP "would have been" given the assumed conception date; the real LMP is just a different anchor.
Can I use this to plan a pregnancy? +
Yes — it is one of the most common uses. Enter your target due date (often a month with personal or work-schedule significance) and see when conception would need to occur. From there, use the <a href="/app/ovulation-calculator/">Ovulation Calculator</a> to identify the matching fertile window in your cycle. Realistic expectation-setting: even healthy couples have roughly a 20% chance of conception per cycle, so plan for several months of trying around the target window.
Why does the calculator show a "conception window" rather than a single conception date? +
Because conception itself spans a window, not a single moment. Sperm survive in the reproductive tract for up to 5 days under good cervical-mucus conditions. The egg is viable for 12–24 hours after ovulation. So if intercourse happened 3 days before ovulation, fertilization may have occurred 3 days later when the egg was released. The "conception window" is the practical answer to "when did this pregnancy start" — typically a 5–6 day band centered on the estimated ovulation day.