Pregnancy Week Calculator

Formula

days since LMP ÷ 7

Worked example

LMP Jan 1; as of Jan 29 — 4 weeks

Quick answer

What does the Pregnancy Week Calculator do?

Estimate gestational week from the first day of the last menstrual period and a selected date.

What is the Pregnancy Week Calculator?

Estimate gestational week from the first day of the last menstrual period and a selected date. Use it when you have the required inputs and want a deterministic, repeatable result with the calculation method visible.

How to use this calculator

Enter the values requested by the tool, keep units and time periods consistent, resolve any validation message, calculate, then review the primary answer together with the displayed formula and supporting values.

Formula and method

This calculator uses the deterministic formula days since LMP ÷ 7. Enter the required values, then review the displayed substitution and result breakdown to verify the calculation.

Worked example

LMP Jan 1; as of Jan 29 4 weeks

Common mistakes to avoid

Check units, decimal points, percentages, signs, date/time order, selector choices, and rounding. For important decisions, verify the result against an independent source or qualified professional when appropriate.