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Last period conception date

Last period conception date: calculation method, required inputs, worked check, limitations and a link to Gestational Age Calculator. Reviewed 24 September 2...

By Uddhav Gopani, FounderPublished 25 September 2026Reviewed 25 September 2026

For “Last period conception date”, the linked Gestational Age Calculator is relevant because Estimate gestational age in weeks and days from the last menstrual period, with the estimated due date.

What this specific question is asking

Most wrong answers here are setup errors, not math errors. Confirm first day of last menstrual period (lmp) and as of date (optional, defaults to today) — units, periods and definitions — before you touch Gestational Age Calculator, and keep the output's unit attached when you use it.

Use the example (As a worked reference for “Last period conception date”, Worked…) as a calibration run: reproduce it first, then change only first day of last menstrual period (lmp). The output should track the change predictably; if it does not, the inputs are on mismatched bases.

Inputs to verify

Verify first day of last menstrual period (lmp) before calculation; if this value uses a different unit, period or definition, convert or restate it first.
Verify as of date (optional, defaults to today) before calculation; if this value uses a different unit, period or definition, convert or restate it first.

Step-by-step check

Enter the first day of the last menstrual period (LMP).
Optionally enter an “as of” date, or leave it blank for today.
Press Calculate to see gestational age in weeks and days.
Use ultrasound dating from your provider to confirm.

Worked example and sanity check

As a worked reference for “Last period conception date”, Worked example If today is 20 weeks and 3 days from the last menstrual period, the pregnancy is at 20w3d — counted in completed weeks plus days.

Interpreting the result

So what does the number mean? In the worked example the calculation produced 20 weeks and 3 days from the last menstrual period, the preg. A result in that range is plausible for this kind of input; if your result lands far from it, revisit first day of last menstrual period (lmp) first — it is the input most often entered on the wrong basis. Read the magnitude before the decimals: a result ten times too large or small almost always means a unit or period slipped, not that the formula failed.

Reviewed 25 September 2026

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Use the Gestational Age Calculator

Open Gestational Age Calculator

Common questions

Which inputs matter most for last period conception date?

The key inputs are First date, Second date. Match their units and periods before using Date Calculator.

How can I check an answer for last period conception date?

Run the example exactly as written, then vary first day of last menstrual period (lmp) alone. A result that tracks that change predictably tells you the setup — units, periods, definitions — is correct.

When should I recalculate last period conception date?

Recalculate whenever any input's basis changes: a new measurement, a revised rate or threshold, a different period convention, or an updated policy figure.