Conception Date Calculator
Inputs
| Start from | Due date |
|---|---|
| Date | Feb 5, 2027 |
| Cycle length | 28 |
Conception Date Calculator
Estimate the likely date of conception and fertile window from an estimated due date or the first day of the last menstrual period.
Inputs
Results
Enter a value to see results.
Details
Conception most likely occurred around ..., within a window of ... to .... This corresponds to an estimated due date of ....
Conception Date
The conception date calculator estimates when conception most likely occurred, working backward from either an estimated due date or the first day of the last menstrual period (LMP). It is the reverse of a due-date estimate: rather than projecting forward to a birth date, it locates the point at which pregnancy is taken to have begun. People consulting medical records, reconstructing a timeline, or simply curious about the probable date of conception use it to convert a known due date or period date into an estimated conception date and the surrounding fertile window.
How conception is estimated
Two reference points are commonly available, and the calculator accepts either.
Working from an estimated due date, conception is placed about 266 days earlier. Pregnancy is conventionally measured as 280 days (40 weeks) from the first day of the last period, but ovulation and conception occur roughly two weeks into that count, leaving about 266 days (38 weeks) of gestation from conception to birth:
Dc=Ddue−266 daysWorking from the last period, conception is placed at ovulation, assumed to occur about 14 days before the next period would have begun. For a cycle of length days, that is:
Dc=DLMP+(L−14) daysFor the textbook 28-day cycle this is 14 days after the period begins. The corresponding due date in this mode follows Naegele's rule, adjusted for cycle length:
Ddue=DLMP+280+(L−28) daysWorked example
Suppose the last period began on 11 March 2026 and the average cycle is 31 days. Ovulation is estimated at seventeen days (thirty-one minus fourteen) after the period began, placing conception on 28 March 2026. The fertile window spans 25 March to 30 March, reflecting the days on which conception was plausible. The estimated due date is two hundred eighty-three days (two hundred eighty plus the three extra cycle days) after 11 March 2026, or 19 December 2026 — three days later than a 28-day cycle would predict, because the longer cycle pushes ovulation later.
The fertile window
A single date overstates the precision of the method. Sperm can survive several days in the reproductive tract, and the exact timing of ovulation varies between and within individuals. The calculator therefore reports a window from about three days before to two days after the central estimate, marking the span during which conception could reasonably have taken place.
Interpretation and limits
These estimates rest on two assumptions: that ovulation occurs about 14 days before the next period, and that cycles are reasonably regular. Both can fail. Cycles vary month to month, ovulation can shift, and a due date derived from an uncertain LMP carries that uncertainty into the conception estimate. Early ultrasound dating measures the embryo directly and is generally more accurate, particularly when cycles are irregular. The figures here are best read as a probable range rather than an exact day.
For the forward calculation, see the Pregnancy Due Date Calculator. To estimate the fertile days within an upcoming cycle, see the Ovulation Predictor.
Frequently Asked Questions (FAQ)
How is the conception date estimated?
From an estimated due date, conception is placed about 266 days (38 weeks) earlier, since pregnancy is taken to last roughly 38 weeks from conception. From the first day of the last menstrual period, conception is placed about 14 days later for a 28-day cycle, on the assumption that ovulation occurs around 14 days before the next period.
Why is a window shown instead of a single day?
Sperm can survive in the reproductive tract for several days, and the exact timing of ovulation varies. The window spans about three days before to two days after the central estimate to reflect the period during which conception was plausible.
How does cycle length change the result?
In last-period mode, ovulation is assumed to occur about 14 days before the next period. A longer cycle places ovulation, and therefore conception, later; a shorter cycle places it earlier. The estimated due date shifts by the same number of days the cycle differs from 28.
Is this more accurate than an ultrasound?
No. These estimates assume average ovulation timing and cycle regularity. Early ultrasound dating measures the embryo directly and is generally more precise, particularly when cycles are irregular or ovulation timing is uncertain.
Disclaimer
This calculator provides an estimate based on average ovulation timing and cycle length. Actual conception can vary by several days, and early ultrasound dating is more precise. It does not constitute medical advice.