CalculationTime

Conception Date Calculator

Estimate conception date and fertile-window timing from due date, last menstrual period or ultrasound gestational age, with clear medical limitations.

Live math canvas

Your numbers, formula and explanation together

Conception Date Calculator: 2026-01-08. due 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 applied

The exact method behind this answer

CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.

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.
  1. Apply the formulaFrom 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.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

Your live breakdown

Current inputs in the calculation

These values come from the controls above and update when the calculator changes.

Calculation method
Due date
Choose the date basis you have.
Known date year
2,026
Due date, LMP date, or ultrasound scan date year.
Known date month
10
Month number for the known date.
Known date day
1
Day of month for the known date.
Ultrasound gestational weeks
8 weeks
Use only for ultrasound method.
Extra gestational days
0 days
Use only for ultrasound method.

Resulting answer

2026-01-08

due 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

Answer
2026-01-08
Live support
due 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

Assumptions used

What this answer assumes

Best for pregnancy date orientation and fertility-window discussion, not diagnosis or legal proof.

  • 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.

Master’s Tip

How to use the result well

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.

Printable record

What belongs in the saved calculation

Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.

Calculation method
Due date
Choose the date basis you have.
Known date year
2,026
Due date, LMP date, or ultrasound scan date year.
Known date month
10
Month number for the known date.
Known date day
1
Day of month for the known date.

Embeddable calculator

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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.

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

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

These cards are resolved from calculator data, category context and live-route status so the page links to useful next steps without hand-built link clutter.

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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