π Reverse Due Date Calculator
From due date back to conception.
For informational purposes only, not medical advice. Consult a healthcare professional.
Pregnancy is dated 280 days from the last menstrual period but only about 266 days from conception, because ovulation happens roughly two weeks into the cycle. Working backward from the due date therefore gives two answers: the last period about 280 days earlier and conception about 266 days earlier, usually expressed as a window of a few days. This calculator does that reverse math for curiosity, records, and paternity questions, with the honest precision limits stated.
How to Use This Calculator
Enter the due date from the ultrasound or Naegele calculation. The calculator subtracts back to the likely conception window and last period.
Enter the due date. First-trimester ultrasound dates are the most accurate anchor.
Read the estimated conception date, given as a small window since fertilization timing varies.
Read the estimated last period date for the record. Both are estimates, good to within a few days.
How the Math Works
Subtract 266 days from the due date for the estimated conception date, presented as a 5-day window centered on that date since ovulation and fertilization timing vary. Subtract 280 days for the estimated first day of the last menstrual period. These assume the standard 28-day cycle with ovulation at day 14; irregular cycles shift conception within the window. Ultrasound dating in the first trimester is accurate to about plus or minus 5 days, so the window honestly reflects the method precision.
Worked Example
With a due date of June 8, 2027, conception most likely happened around Sep 15, 2026, and the last period began around Sep 1, 2026. The window spans a few days because ovulation does not follow a stopwatch. Anyone using this for paternity timing should note the plus-or-minus several days of honest uncertainty in every direction.
Reading Your Result
Use the conception window for what it is: an estimate good to several days. For paternity questions it narrows the timeframe but cannot name a day; DNA testing answers what calendars cannot. For records and curiosity it is perfectly adequate. If the pregnancy was dated by later ultrasound, widen the mental window, since second-trimester dating carries plus-or-minus two weeks.
| Anchor | Days before due date | Precision |
|---|---|---|
| Conception | 266 | Window of days |
| Last period | 280 | Plus/minus days |
Limitations
The 266-day figure assumes ovulation on cycle day 14; real ovulation varies, especially in irregular cycles. Ultrasound dating has its own error bars that widen with gestational age. This tool estimates for information; it cannot determine paternity or precise fertilization timing.
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Related tools: pregnancy due date calculator ovulation calculator pregnancy test timing calculator.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How do I calculate my conception date from my due date?
Subtract 266 days from the due date. That is the average interval from fertilization to delivery. Present it as a window of a few days rather than a single date, since ovulation timing varies.
Why 266 days and not 280?
Because the 280-day pregnancy calendar starts at the last menstrual period, about two weeks before ovulation. Conception happens near ovulation, so it sits about 266 days before the due date. Both numbers describe the same pregnancy from different start lines.
How accurate is the conception window?
Plus or minus several days with first-trimester ultrasound dating, wider with later dating or irregular cycles. Sperm survive up to 5 days, which adds its own spread. Treat the window as a timeframe, not an appointment.
Can this determine paternity?
No. It narrows the window but cannot distinguish between encounters days apart, and biology varies. Only DNA testing determines paternity. Calendar math has ended up wrong in courtrooms before.
What if my cycles are irregular?
The estimate gets looser because ovulation day is uncertain. The calculator still gives the standard-math answer, but widen the window mentally. Early ultrasound dating helps more than calendar arithmetic in irregular cycles.
Does IVF change the math?
Yes, precisely: with IVF the fertilization date is known exactly, so no estimation is needed. The due date itself is then calculated forward 266 days from the retrieval or transfer date with adjustments. This calculator is for spontaneous conceptions.
What is Naegele rule?
The classic due-date formula: last period plus one year, minus 3 months, plus 7 days. It encodes the 280-day gestation. The reverse calculator here is Naegele run backward with the conception offset.
Why do doctors date from the last period instead of conception?
Because the last period is a memorable, observable event while conception is usually unobserved. The convention predates ultrasound and stuck because it works consistently. First-trimester ultrasound now refines it.
Can conception happen outside the window?
Late ovulation, long sperm survival, and dating errors can all push the true date to the edges or beyond. The window covers the likely range, not every possibility. Outliers exist in both directions.
Should I share this with my doctor?
The dates are useful background for the first prenatal visit, but the clinician will establish dating with ultrasound. Bring the due date you were given; the reverse math is context, not a competing opinion.