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How to calculate my pregnancy weeks and months?

How to calculate my pregnancy weeks and months: calculation method, required inputs, worked check, limitations and a link to Gestational Age Calculator. Revi...

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

For “How to calculate my pregnancy weeks and months”, 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

Before running Gestational Age Calculator, write down what first day of last menstrual period (lmp) and as of date (optional, defaults to today) mean for your case — including each unit and time period. The calculator multiplies whatever it is given; the setup decides whether the answer is usable.

Test the setup before trusting the output: rerun the example with a doubled first day of last menstrual period (lmp) and see whether the result behaves as the formula implies. Direction first, precision second.

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 “How to calculate my pregnancy weeks and months”, 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

Interpretation starts with magnitude. The worked example landed at 20 weeks and 3 days from the last menstrual period, the preg; compare yours before trusting the decimals. If the two disagree wildly, the setup — usually first day of last menstrual period (lmp)'s unit or period — is the suspect. 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

Verify any current tax threshold, lender rate, provider rule or health guidance with the authoritative source before acting.

Use the Gestational Age Calculator

Open Gestational Age Calculator

Authoritative references

ACOG pregnancy dating guidance

Common questions

Which inputs matter most for how to calculate my pregnancy weeks and months?

The key inputs are First day of last menstrual period (LMP), Average cycle length (days). Match their units and periods before using Pregnancy Due Date Calculator.

How can I check an answer for how to calculate my pregnancy weeks and months?

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 how to calculate my pregnancy weeks and months?

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