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Conception Calculator

Updated Sep 2026 Used 0 times
Choose the date and dating method you know.
Used in LMP mode.
Used in due-date mode.
Used when conception is known clinically.
Date of the scan used for dating.
Completed gestational weeks recorded at the scan.
Additional days after completed weeks.
Use the transfer date recorded by the fertility clinic.
Used only to adjust LMP dating.

Guest calculations stay on this device. Signed-in results sync privately.

Your result

Conception

Enter your values, then calculate to see a verified result.

Inputs usedReview the information used for this result.
Full calculation and sourcesLMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Versioned calculationFormula v1.0.0

Formula definitionUnit-normalized calculationFormula v1.0.0

What this result means

Estimate conception timing from LMP, due date, conception date, ultrasound dating or IVF transfer and show a wider intercourse window without claiming paternity evidence.

Method used

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Important limitation

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Result actions
1
Enter the inputsUse values with the units and definitions shown beside each field.
2
Apply the methodLMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.
3
Inspect the bridgeReview the primary result and supporting outputs before using it.
4
Check limitationsConfirm the assumptions, date, jurisdiction or population match your use case.

Interpretation

A date estimate cannot prove which intercourse caused pregnancy or establish paternity.

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Use this result

Keep exploring

3 available tools
Pregnancy Due Date CalculatorEstimate a due date from LMP, conception, ultrasound dating, a known due date or IVF transfer with ACOG-aligned limitations.Gestational Age CalculatorCalculate gestational weeks and days from an LMP or known due date to a selected reference date.Ovulation CalculatorEstimate ovulation and a broader possible conception window from LMP and cycle length with an explicit variability warning.
Public verification recordFormula v1.0.0
Review scope
Conception Calculator Formula, Input Validation, Edge Behavior And Result Presentation.
Review team
CalculatorGeek Expert Review Team
Verified
2026-09-21
Next source review
2027-09-21
Automated fixtures
1 cases

Review method

Package reconciliation, primary-source mapping, competitor feature comparison, deterministic fixtures, PHP/JavaScript parity and responsive preview QA.

Known limitations

  • Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.
  • Changing legal, medical, exam or statistical conventions must be checked at the source.

Primary sources

Latest update

2026-09-21 ·

On this page

Direct answer

Estimate conception timing from LMP, due date, conception date, ultrasound dating or IVF transfer and show a wider intercourse window without claiming paternity evidence.

Continue with the Pregnancy Due Date Calculator when that method matches your next question.

Continue with the Gestational Age Calculator when that method matches your next question.

Formula and methodology

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

A date estimate cannot prove which intercourse caused pregnancy or establish paternity.

Continue with Pregnancy Due Date Calculator or Gestational Age Calculator for the next calculation.

Worked example

A date estimate cannot prove which intercourse caused pregnancy or establish paternity.

The calculator retains working precision and rounds only the displayed values.

Interpret the result carefully

Method boundary: Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Inputs, boundary behavior and JavaScript/PHP parity are covered by deterministic release tests.

Use the result for the named task only. Do not stretch it into a legal, clinical, safety or causal conclusion the formula does not support.

Inputs, outputs and result meaning

Calculates estimated conception, possible intercourse-window start, possible intercourse-window end, and estimated due date from calculate from, first day of LMP, estimated due date, and known conception date with visible method, validation, precision, and limitations.

Inputs

Calculate from
Choose the date and dating method you know. Available choices are Last menstrual period, Estimated due date, Known conception date, Ultrasound gestational age, IVF day-3 embryo transfer, and IVF day-5 embryo transfer.
First day of LMP
Used in LMP mode. The supported range is 1900-01-01 through 2100-12-31.
Estimated due date
Used in due-date mode. The supported range is 1900-01-01 through 2100-12-31.
Known conception date
Used when conception is known clinically. The supported range is 1900-01-01 through 2100-12-31.
Ultrasound date
Date of the scan used for dating. The supported range is 1900-01-01 through 2100-12-31.
Gestational weeks on scan
Completed gestational weeks recorded at the scan. The supported range is 0 through 42.
Additional gestational days
Additional days after completed weeks. The supported range is 0 through 6.
Embryo transfer date
Use the transfer date recorded by the fertility clinic. The supported range is 1900-01-01 through 2100-12-31.
Typical cycle length
Used only to adjust LMP dating. The supported range is 21 through 45.

Outputs

Estimated conception
This is the primary result. The visible value is displayed without decimal places.
Possible intercourse-window start
The visible value is displayed without decimal places.
Possible intercourse-window end
The visible value is displayed without decimal places.
Estimated due date
The visible value is displayed without decimal places.

The calculator applies the documented method after validating and normalizing the inputs. Keep the result label, unit, selected mode, and stated limitations together when sharing the answer.

Frequently asked questions

What method does this calculator use?

LMP uses 280 days plus cycle adjustment; conception uses 266 days; ultrasound back-calculates from recorded gestational age; day-3 and day-5 IVF transfers use 263 and 261 days.

How should I round the result?

Keep full precision during the calculation and round only the final display unless a governing standard says otherwise.

What is the main limitation?

Calendar dating produces an estimated center and possible intercourse window. It cannot identify a precise fertilization event, prove paternity or replace a documented obstetric estimate.

Can I reproduce the answer?

Yes. The formula, inputs, supporting outputs and worked example are displayed on the page.

Sources

Source research refreshed September 21, 2026. No rankings, review credentials or official affiliation are implied.