Formula
From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.
Health & Body
Conception Date Calculator
Estimate conception date and fertile-window timing from due date, last menstrual period or ultrasound gestational age, with clear medical limitations.
Health & Body
Conception Date Calculator
Live answer2026-01-08due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01
Live result2026-01-08due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01
Formula used
From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.
This is the method behind the answer, so the result can be checked rather than simply trusted.Visual grid
This number is one point on a larger pattern
Conception Date is not just a final answer. It is a step on a line: before and after, input and output, assumption and result.
Micro-timehours, minutes, shiftsHuman scaledays, weeks, projectsMacro-timemonths, years, calendars
InputFormulaResult
2026-01-08CalculationTime keeps the path visible: the input, the method and the final number belong together.
CalculationTime
Conception Date Calculation Report
2026-01-08due date minus 266 days · fertile-window estimate 2026-01-03 to 2026-01-09 · estimated LMP 2025-12-25 · estimated due date 2026-10-01
Inputs
- Calculation method
- Due date
- Known date year
- 2,026
- Known date month
- 10
- Known date day
- 1
- Ultrasound gestational weeks
- 8 weeks
- Extra gestational days
- 0 days
Method
From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.
- If the due date is 1 October 2026, estimated conception is about 266 days earlier, with a fertile-window estimate around the five days before that date through ovulation.
Assumptions
- The estimate is not proof of biological parentage or exact conception timing.
- Ovulation, implantation and dating can vary by cycle length, health context and scan timing.
- Medical dating should be confirmed with a qualified clinician.
- Ultrasound gestational age is treated as completed weeks plus days on the scan date.
Notes
Use this space on the printed report for client, supplier, classroom, job-location, measurement, quote or approval notes.
Worked example
If the due date is 1 October 2026, estimated conception is about 266 days earlier, with a fertile-window estimate around the five days before that date through ovulation.
Professional note
Master’s Tip: treat this as a date window, not a courtroom answer. Pregnancy dating is strongest when LMP, scan results and clinician assessment agree.
Regional and unit assumptions
General obstetric dating arithmetic: due date about 280 days after LMP and conception about 266 days before due date.
Assumptions and limitations
- The estimate is not proof of biological parentage or exact conception timing.
- Ovulation, implantation and dating can vary by cycle length, health context and scan timing.
- Medical dating should be confirmed with a qualified clinician.
- Ultrasound gestational age is treated as completed weeks plus days on the scan date.
Methodology & Accuracy
How this calculator is checked
CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.
Formula used
From due date: conception ≈ due date − 266 days. From LMP: conception ≈ LMP + 14 days. From ultrasound: estimated LMP = scan date − gestational age; conception ≈ estimated LMP + 14 days.
Standard or basis
General obstetric dating arithmetic: due date about 280 days after LMP and conception about 266 days before due date.
Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.Master's Tip
Master’s Tip: treat this as a date window, not a courtroom answer. Pregnancy dating is strongest when LMP, scan results and clinician assessment agree.
Related calculators
Questions
Can conception date be exact?
Usually no. Fertilization, ovulation timing and implantation vary, so a window is safer than a single exact date.
Why subtract 266 days from due date?
A common pregnancy due date is about 280 days from LMP, and ovulation/conception is often estimated about 14 days after LMP.
Can this identify a father?
No. Use medical and legal advice for paternity questions.
Calculation note
Pregnancy dates are conventionally measured from LMP even though conception happens later. A useful calculator names that convention and its uncertainty.
Sources and further readingACOG — Methods for Estimating the Due Date