Work backward from a known due date to estimate your last menstrual period date, conception date, and when your second and third trimesters started or will start.
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Due dates are conventionally calculated as 280 days (40 weeks) after the last menstrual period (LMP), so this calculator reverses that: subtracting 280 days from your due date to estimate your LMP. From there, it adds roughly 14 typical days to estimate a conception date, and adds 13 and 27 weeks respectively to estimate when the second and third trimesters began or will begin.
For example, for a due date of March 15, 2027: the estimated LMP is 280 days earlier, June 8, 2026. Estimated conception is 14 days after that, June 22, 2026. The second trimester is estimated to start 13 weeks (91 days) after the LMP, September 7, 2026, and the third trimester 27 weeks (189 days) after the LMP, December 14, 2026.
Unlike our Pregnancy Week Calculator, this tool doesn't reference today's date at all, it's purely a reconstruction of your estimated pregnancy timeline from a single known due date. This is useful right after receiving a due date from a doctor or ultrasound and wanting to see the full estimated timeline at once, rather than tracking ongoing weekly progress.
Confusing this reverse calculator with the forward-calculating Pregnancy Due Date Calculator, this tool starts from a known due date and works backward to estimate LMP and conception, the other starts from your LMP and works forward to a due date.
Treating the reconstructed LMP and conception dates as precise facts rather than estimates, they're derived from the standard 280-day/14-day assumptions and can be off by a week or more if your actual cycle or ovulation timing differed from average.
Plugging in an actual birth date instead of the original estimated due date, since most babies aren't born exactly on their due date, doing this shifts every reconstructed date (LMP, conception, trimester starts) away from what was actually true during the pregnancy.
If your due date came from an ultrasound rather than your own LMP tracking (common, and often considered more accurate, especially with irregular cycles), you may not have an exact LMP date on hand, this calculator reconstructs an estimate from the due date instead.
It's a typical estimate assuming conception occurred about 14 days after LMP, which is standard for an average cycle, but actual ovulation timing varies by individual and cycle, so treat this as an approximation.
They share the same underlying 280-day math, but this calculator focuses on reconstructing your full estimated timeline (LMP, conception, trimester start dates) as a reference, while the Pregnancy Week Calculator focuses on your current week, day and trimester relative to today.
Pregnancy Week Calculator
Find your current pregnancy week and trimester from either your LMP or your due date.
Fertility Window Calculator
Estimate your fertile window using the calendar method, accounting for cycle-length variability.
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