Work backwards from a known due date or birth date to estimate the likely conception date, fertile window, and last menstrual period.
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Health — Pregnancy
A reverse due date calculator works backwards from a known or estimated due date (or an actual birth date) to estimate when conception likely occurred and what the fertile window would have been. It applies Naegele's rule in reverse — rather than calculating a due date from the last period, it estimates the last period and conception window from the due date.
Naegele's rule states that the estimated due date (EDD) equals the last menstrual period (LMP) plus 280 days. Working backwards, the estimated LMP is simply EDD minus 280 days, and the estimated conception date is EDD minus 266 days (38 weeks before delivery).
Estimated LMP = EDD − 280 days
Backwards Naegele's rule
Estimated conception = EDD − 266 days
Approximately 14 days after estimated LMP
Conception requires sperm to be present in the fallopian tubes at the time of ovulation. Sperm can survive up to five days, so the fertile window extends roughly five days before ovulation to one day after.
The intercourse window shown in this calculator represents the three days before estimated ovulation to one day after — the period most commonly associated with successful conception attempts.
Frequently asked questions
The estimate assumes a standard 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, actual conception would have occurred earlier or later than the estimate. The true conception window has a natural uncertainty of several days.
Yes. Most births occur close to the due date, so using the actual birth date gives a similar estimate. However, premature or post-term births will shift the estimated conception window accordingly.
Related
These related calculators come from the same leaf category, nearby sibling categories, or the same top-level topic.
Estimate your pregnancy due date from your last period or conception date, with gestational age and trimester timing.
Estimate the expected delivery date from last period, conception date, ultrasound scan, or IVF embryo transfer.
Estimate conception date and fertile window from your last menstrual period and cycle length, or work backwards from a known due date.
Estimate gestational age in weeks and days from last menstrual period, due date, or conception date, with trimester and due date.
