About the reverse due date calculator
A reverse due date calculator works backward from a known or estimated due date to find the dates that produced it: the estimated first day of the last menstrual period (LMP) and the likely date of conception. It uses the same arithmetic that produced the due date in the first place, run in reverse, so the numbers stay consistent with how due dates are normally calculated.
The formula: Naegele's rule, reversed
Most due dates are set using Naegele's rule: LMP date plus 280 days (40 weeks). This calculator reverses that single step - subtracting 280 days from the due date to recover the estimated LMP. From there, the estimated conception (ovulation) date is the LMP plus the cycle length minus 14 days, since the second half of the cycle (the luteal phase, from ovulation to the next period) is fairly constant at about 14 days even when overall cycle length varies. A 28-day cycle places ovulation on day 14; a 32-day cycle places it on day 18.
Reading gestational age and the full-term window
Gestational age is conventionally counted in weeks and days from the LMP, not from the conception date - so "8 weeks pregnant" means roughly 8 weeks since the LMP, or about 6 weeks since conception. The calculator reports today's gestational age from that same LMP-based count. The American College of Obstetricians and Gynecologists (ACOG) defines a full-term pregnancy as 39 weeks 0 days through 40 weeks 6 days from the LMP, inside the broader 37-week-0-day to 41-week-6-day window that also covers early term and late term.
Why the estimate has a margin of error
Both the reversed LMP date and the conception date are estimates, not measurements. They assume a due date calculated by Naegele's rule and a textbook 14-day luteal phase. Actual ovulation timing varies from cycle to cycle and person to person, and a due date confirmed by early ultrasound can differ from the LMP-based date by several days. Treat the conception date as a several-day window rather than a single exact day.
When to consult a professional
This calculator performs standard obstetric date arithmetic for planning and record-keeping. It does not replace prenatal care. For confirmation of your due date, conception timing, or any pregnancy-related decision, consult your obstetrician, midwife, or other qualified healthcare provider, who can incorporate ultrasound dating and your individual history.