Reverse Due Date Calculator
How the math works backward
A due date is normally estimated as the first day of your last period (LMP) plus 280 days — Naegele's rule. Reversing that: LMP = due date − 280 days.
Conception typically happens around ovulation, roughly 14 days after the LMP, so it's estimated as due date minus 266 days. If you conceived via IVF, your IVF due date calculator will be more precise since it's based on your actual transfer date.
Conception month → birth month, at a glance
| Conceive in | Baby due around |
|---|
Things to know about working backward
This tool assumes a typical 28-day cycle with ovulation around day 14, since that's the same assumption a due date was likely built on in the first place. If your actual cycles run shorter or longer than 28 days, your real conception date and LMP probably shifted earlier or later than what's shown here — the further your cycles are from 28 days, the bigger that gap tends to be.
If your due date itself was set or adjusted by an early ultrasound rather than your last period, that's the more reliable number to work backward from anyway, since ultrasound dating doesn't depend on cycle-length assumptions the way LMP-based dating does.
It's also worth remembering that a due date is an estimate of when labor might begin on its own, not a guarantee — only a small share of babies are actually born on their exact due date, and full term is generally considered anywhere from 37 to 42 weeks. If a birth was induced or scheduled (for example, a planned C-section) before labor started naturally, the actual birth date may not reflect when conception or the last period genuinely occurred, which matters if you're using someone's actual birth date rather than a due date to estimate backward.
These estimates are meant for general curiosity and planning — for anything that requires precision (like legal or medical documentation), your prenatal records and ultrasound reports are the authoritative source.
The single conception date shown here is really shorthand for a short window, not one exact day — ovulation and the days just before it are all biologically plausible conception points. If you'd rather see that as a range instead of a single date, our Conception Calculator shows a possible window alongside the most likely date, using the same underlying math.
Sources
The 280-day (LMP) and 266-day (conception) conventions used here reflect standard obstetric dating practice. For more background on pregnancy dating, see the American College of Obstetricians and Gynecologists (ACOG) and Mayo Clinic.